
m^ 



^^ 



foipglitN'' I ^ ^^ 



CIIEXRIGHI OEPOSm 














iBiiHB 



.^c^' 

>^? ^^4.- ' 






:%^" 
>>-!* 



NCE Li 



N'-CHIEF 

KRETT P. SERVISS 



AUTHORS 

HIPPOLflE GRUENER A. RUSSELL BOND 

LOOMIS HaS^EMEYER ERNEST G. MARTIN 
!HOVVN^ ROBERT CHENAULT GIVLER 
'LL ^VILFRED MASON BARTON 
( OTT ^ ERNEST J.'sTREUBEL 
AYL(^l DAVID TODD 

S:S FJ^g?HUGH iAI.MAN 
HOBfgJBEACM 



■y.D IX gXTEEN VOLUMES 
WITH A); "iV oFsCIENCE. GLOSSARIES 

u p 



IL^J^RATED 



P*, 



if 



ON COMPANY 

. RK 



Popular Science Library 

EDITOR-IN-CHIEF 

GARRETT P. SERVISS 

AUTHORS 

WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND 

D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN 

ARTHUR SELWYN-BROVVN ROBERT CHENAULT GIVLER 

ERNEST INGERSOLL WILFRED MASON BARTON 

WILLIAM B. SCOTT ERNEST J. STREUBEL 

NORMAN TAYLOR DAVID TODD 

CHARLES FITZHUGH TALMAN 

ROBIN BEACH 



ARRANGED IN SIXTEEN VOLUMES 

WITH A HISTORY OF SCIENCE, GLOSSARIES 

AND A GENERAL INDEX 



ILLUSTRATED "^ 



VOLUME TWO I 



P. F. COLLIER & SON COMPANY 

NEW YORK 



Copyright 1922 / 

By p. F. Collier & Son Company \^ 

manufactured in u. s. a. 



DEC 20 '22 . 



I 



©CU692479 



ASTRONOMY 

The Science of the Heavenly Bodies 



DAVID TODD ? 
Director of Observatory, Amherst College 



P. F. COLLIER & SON COMPANY 
NEW YORK 



A 



'\ 



6> 






PREFACE 

SIR WILLIAM ROWAN HAMILTON, the emi- 
nent mathematician of Dublin, has, of all writers 
ancient and modern, most fittingly characterized 
the ideal science of astronomy as man's golden chain 
connecting the heavens to the earth, by which we 
"learn the language and interpret the oracles of the 
universe/' 

The oldest of the sciences, astronomy is also the 
broadest in its relations to human knowledge and 
the interests of mankind. Many are the cognate 
sciences upon which the noble structure of astronomy 
has been erected : foremost of all, geometry and the 
higher mathematics, which tell us of motions, mag- 
nitudes and distances; physics and chemistry, of 
the origin, nature, and destinies of planets, sun, and 
star; meteorology, of the circulation of their at- 
mospheres; geology, of the structure of the moon's 
surface; mineralogy, of the constitution of mete- 
orites; while, if we attack, even elementally, the 
fascinating, though perhaps forever unsolvable, 
problem of life in other worlds, the astronomer must 
invoke all the resources that his fellow biologists 
and their many-sided science can afford him. 

The progress of astronomy from age to age has 
been far from uniform — rather by leaps and bounds : 
from the earliest epoch when man's planet earth 
was the center about which the stupendous cosmos 
wheeled, for whom it was created, and for whose 
edification it was maintained — down to the modern 



4 PREFACE 

age whose discoveries have ascertained that even 
our stellar universe, the vast region of the solar 
domain, is but one of the thousands of island uni- 
verses that tenant the inconceivable immensities of 
space. 

Such results have been attainable only through 
the successful construction and operation of monster 
telescopes that bring to the eye and visualize on 
photographic plates the faintest of celestial objects 
v/hich were the despair of astronomers only a few 
years ago. 

But the end is not yet; astronomy to-day is but 
passing from infancy to youth. And with new and 
greater telescopes, with new photographic processes 
of higher sensitivity, with the help of modern in- 
vention in overcoming the obstacle of the air — ^that 
constant foe of the astronomer — who will presume 
to set down any limit to the leaps and bounds of 
astronomy in the future? 

So rapid, indeed, has been the progress of as- 
tronomy in very recent years that the present is 
especially favorable for setting forth its salient 
features; and this book is an attempt to present 
the wide range of astronomy in readable fashion, 
as if a story with a definite plot, from its origin 
with the shepherds of ancient Chaldea down to 
present-day ascertainment of the actual scale of the 
universe, and definite measures of the huge volume 
of supersolar giants among the stars. 

David Todd 

Amherst College Observatory 
November, 1921 



CONTENTS 



CHAPTER PAGE 

I. Astronomy a Living Science .... 9 

II. The First Astronomers 19 

III. Pyramid, Tomb, and Temple .... 23 

IV. Origin of Greek Astronomy .... 27 
V. Measuring the Earth — Eratosthenes . 30 

VI. Ptolemy and His Great Book ... 33 

VII. Astronomy of the Middle Ages ... 37 

VIII. Copernicus and the New Era .... 42 

IX. Tycho, the Great Observer 45 

X. Kepler, the Great Calculator .... 49 

XI. Galileo, the Great Experimenter ... 53 

XII. After the Great Masters 57 

XIII. Newton and Motion 62 

XIV. Newton and Gravitation 66 

XV. After Newton 73 

XVI. Halley and His Comet 83 

XVII. Bradley and Aberration 90 

XVIII. The Telescope 93 

XIX. Reflectors — Mirror Telescopes .... 102 

XX. The Story of the Spectroscope . . . Ill 

XXI. The Story of Astronomical Photography 125 

XXII. Mountain Observatories 139 

XXIII. The Frogp-am of a Great Observatory . 152 

XXIV. Our Solar System 162 

XXV. The Sun and Observing It 165 

XXVI. Sun Spots and Prominences .... 174 

XXVII. The Inner Planets 189 

XXVIII. The Moon and Her Surface .... 193 

XXIX. Eclipses of the Moon 206 

XXX. Total Eclipses of the Sun 209 

XXXI. The Solar Corona 219 

5 



6 CONTENTS 

CHAPTER PAGE 

XXXII. The Ruddy Planet 227 

XXXIII. The Canals of Mars 235 

XXXIV. Life in Other Worlds 242 

XXXV. The Little Planets 254 

XXXVI. The Giant Planet 260 

XXXVII. The Ringed Planet 264 

XXXVIII. The Farthest Planets 267 

XXXIX. The Trans-Neptunian Planet ... 270 

XL. Comets — the Hairy Stars 273 

XLI. Where Do Comets Come From? . . . 279 

XLII. Meteors and Shooting Stars .... 283 

XLIII. Meteorites 290 

XLIV. The Universe of Stars 294 

XLV. Star Charts and Catalogues .... 300 

XLVI. The Sun's Motion Toward Lyra . . 304 

XLVII. Stars and Their Spectral Type ... 307 

XLVIII. Star Distances 311 

XLIX. The Nearest Stars 319 

L. Actual Dimensions of the Stars . . 321 

LI. The Variable Stars 324 

LII. The Nov^, or New Stars .... 331 

LIII. The Double Stars 334 

LIV. The Star Clusters 336 

LV. Moving Clusters 341 

LVI. The Two Star Streams 345 

LVII. The Galaxy or Milky Way .... 350 

LVIII. Star Clouds and Nebula 357 

LIX. The Spiral Nebula 361 

LX. Cosmogony ^^^ 

LXI. Cosmogony in Transition 380 



LIST OF ILLUSTRATIONS 

Active Prominence op the Sun, 140,000 Miles High 

Frontispiece 

FACING PAGE 

Nicholas Copernicus 64 

Galileo Galilei 64 

Johann Kepler 65 

Sir Isaac Newton 65 

The Hundred-Inch Reflecting Telescope at Mount 

Wilson 96 

The Forty-Inch Refracting Telescope, Yerkes Ob- 
servatory 96 

150-Foot Tower, Mount Wilson, a Diagram of Tower 

AND Pit 97 

150-Foot Tower — Exterior Viev/ 97 

View Looking Down into the Pit Beneath 150-Foot 

Tower 97 

Mount Wilson Solar Observatory — the 100-Foot 

Dome 128 

Mount Chimborazo, the Best Site in the World for 

AN Observatory 128 

Lick Observatory, Mount Hamilton, California . 129 

Photographing with the 40-Inch Refractor . . 129 

Great Sunspot Group of August 8, 1917 .... 160 

Calcium Flocculi on the Sun 161 

Eclipse of the Moon, with the Lunar Surface Visible 161 

Moon's Surface in the Region of Copernicus . . 192 
South Central Portion of the Moon, at Last 

Quarter 193 

7 



8 LIST OF ILLUSTRATIONS 

FACING PAGB 

Corona of the Sun During an Eclipse .... 224 

Venus, in the Crescent Phase 225 

Mars, Showing Bright Polar Cap 225 

Jupiter, the Giant Planet 256 

Neptune and Its Satellites 256 

Saturn, with Edge of Rings only in View . . , 257 

Saturn, with Rings Displayed to Fullest Extent . 257 

Two Views of Halley's Comet 288 

Swift's Comet, which Showed Remarkable Trans- 
formations .... 288 

Meteor Trail in Field with Fine Nebula . . . 289 

Ring Nebula in Lyra 320 

Dumb-bell Nebula 321 

Star Clouds and Black Holes in Sagittarius . . 352 

Great Nebula in Andromeda 353 



CHAPTER I 

ASTRONOMY A LIVING SCIENCE 

I IKE life itself we do not know when astronomy 
-i began; we cannot conceive a time when it was 
not. Man of the early stone age must have begun 
to observe sun, moon, and stars, because all the 
bodies of the cosmos were there, then as now. With 
his intellectual birth astronomy was born. 

Onward through the childhood of the race he 
began to think on the things he observed, to make 
crude records of times and seasons; the Chaldeans 
and Chinese began each their own system of 
astronomy, the causes of things and the reasons 
underlying phenomena began to attract attention, 
and astronomy was cultivated not for its own sake, 
but because of its practical utility in supplying the 
data necessary to accurate astrological prediction. 
Belief in astrology was universal. 

The earth set in the midst of the wonders of the 
sky was the reason for it all. Clearly the earth 
was created for humanity ; so, too, the heavens were 
created for the edification of the race. All was sub- 
servient to man; naturally all was geocentric, or 
earth-centered. From the savage who could count 
only to five, the digits of one hand, civilized man 
very slowly began to evolve; he noted the progress 
of the seasons; the old records of eclipses showed 
Thales, an early Greek, how to predict their 
happenings, and true science had its birth when 

9 



10 ASTRONOMY TO-DAY 

man acquired the power to make forecasts that 
always came true. 

Few ancient philosophers were greater than 
Pythagoras, and his conceptions of the order of 
the heavens and the shape and motion of the 
earth were so near the truth that we sometimes 
wonder how they could have been rejected for 
twenty centuries. We must remember, however, 
that man had not yet learned the art of measuring 
things, and the world could not be brought into 
subjection to him until he had. To measure he must 
have tools — instruments; to have instruments he 
must learn the art of working in metals, and all 
this took time; it was a slow and in large part 
imperceptible process ; it is not yet finished. 

The earliest really sturdy manifestation of 
astronomical life came with the birth of Greek 
science, culminating with Aristarchus, Hipparchus 
and Ptolemy. The last of these great philosophers, 
realizing that only the art of writing prevents man's 
knowledge from perishing with him, set down all 
the astronomical knowledge of that day in one of 
the three greatest books on astronomy ever written, 
the Almagest, a name for it derived through the 
Arabic, and really meaning "the greatest." 

The system of earth and heaven seemed as if 
finished, and the authority of Ptolemy and his Alma- 
gest were as Holy Writ for the unfortunate cen- 
turies that followed him. With fatal persistence 
the fundamental error of his system delayed the 
evolutionary life of the science through all that 
period. 

But man had begun to measure. Geometry had 
been born and Eratosthenes had indeed measured 
the size of the earth. Tools in bronze and iron were 



ASTRONOMY A LIVING SCIENCE 11 

fashioned closely after the models of tools of stone; 
astrolabes and armillary spheres were first built 
on geometric spheres and circles; and science was 
then laid away for the slumber of the Dark 
Ages. 

Nevertheless, through all this dreary period the 
life of the youthful astronomical giant was main- 
tained. Time went on, the heavens revolved; sun, 
moon, and stars kept their appointed places, and 
Arab and Moor and the savage monarchs of the East 
were there to observe and record, even if the world- 
mind was lying fallow, and no genius had been born 
to inspire anew that direction of human intellect on 
which the later growth of science and civilization 
depends. With the growth of the collective mind 
of mankind, from generation to generation, we note 
that ordered sequence of events which characterizes 
the development of astronomy from earliest peoples 
down to the age of Newton, Herschel, and the 
present. It is the unfolding of a story as if with a 
definite plot from the beginning. 

Leaving to philosophical writers the great funda- 
mental reason underlying the intellectual lethargy 
of the Dark Ages, we only note that astronomy and 
its development suffered with every other depart- 
ment of human activity that concerned the intel- 
lectual progress of the race. To knowledge of every 
sort the medieval spirit was hostile. But with the 
founding and growth of universities, a new era 
began. The time was ripe for Copernicus and a 
new system of the heavens. The discovery of the 
New World and the revival of learning through 
the universities added that stimulus and inspiration 
which marked the transition from the Middle Ages 
to our modern era, and the life of astronomy, long 



12 ASTRONOMY TO-DAY 

dormant, was quickened to an extraordinary de- 
velopment. 

It fell to the lot of Copernicus to write the second 
great book on astronomy, "De Revolutionibus Or- 
bium Ccelestium." But the new heliocentric or sun- 
centered system of Copernicus, while it was the true 
system bidding fair to replace the false, could not 
be firmly established except on the basis of accurate 
observation. 

How fortunate was the occurrence of the new 
star of 1572, that turned the keen intellect of Tycho 
Brahe toward the heavens! Without the observa- 
tional labors of Tycho's lifetime, what would the 
mathematical genius of Kepler have availed in dis- 
covery of his laws of motion of the planets ? 

Historians dwell on the destruction and violent 
conflicts of certain centuries of the Middle Ages, 
quite overlooking the constructive work in progress 
through the entire era. Much of this was of a nature 
absolutely essential to the new life that was to 
manifest itself in astronomy. The Arabs had made 
important improvements in mathematical processes, 
European artisans had made great advances in the 
manufacture of glass and in the tools for working 
in metals. 

Then came Galileo with his telescope revealing 
anew the universe to mankind. It was the north of 
Italy where the Renaissance was most potent, re- 
calling the vigorous life of ancient Greece. Coperni- 
cus had studied here; it was the home of Galileo. 
Columbus was a Genoese, and the compass which 
guided him to the Western World was a product 
of deft Italian artisans whose skill with that of 
their successors was now available to construct the 
instruments necessary for further progress in the 



ASTRONOMY A LIVING SCIENCE 13 

accurate science of astronomical observation. Even 
before Copernicus, Johann Miiller, better known as 
Regiomontanus, had imbibed the learning of the 
Greeks while studying in Italy, and founded an ob- 
servatory and issued nautical almanacs from Nu- 
remberg, the basis of those by which Columbus was 
guided over untraversed seas. 

About this time, too, the art of printing was 
invented, and the interrelation of all the movements 
then in progress led up to a general awakening of 
the mind of man, and eventually an outburst in 
science and learning, which has continued to the 
present day. Naturally it put new life into astron- 
omy, and led directly up from Galileo and his experi- 
mental philosophy to Newton and the "Principia," 
the third in the trinity of great astronomical books 
of all time. 

To get to the bottom of things, one must study 
intimately the history of the intellectual develop- 
ment of Europe through the fifteenth and sixteenth 
centuries. Many of the western countries were ruled 
by sovereigns of extraordinary vigor and force of 
character, and their activities tended strongly to- 
ward that firm basis on which the foundations of 
modern civilization were securely laid. 

Contemporaneously with this era, and following 
on through the seventeenth century, came the 
measurements of the earth by French geodesists, 
the construction of greater and greater telescopes 
and the wonderful discoveries with them by Huy- 
gens, Cassini, and many others. 

Most important of all was the application of 
telescopes to the instruments with which angles are 
measured. Then for the first time man had begun 
to find out that by accurate measures of the heavenly 



14 ASTRONOMY TO-DAY 

bodies, their places among the stars, their sizes and 
distances, he could attain to complete knowledge of 
them and so conquer the universe. 

But he soon realized the insufficiency of the 
mathematical tools with which he worked — ^how un- 
suited they were to the solution of the problem of 
three bodies (sun, earth, and moon) under the New- 
tonian law of gravitation, let alone the problem of 
^-bodies, mutually attracting each the other; and 
every one perturbing the motion of every other one. 
So the invention of new mathematical tools was 
prosecuted by Newton and his rival Leibnitz, who, 
by the way, showed himself as great a man as 
mathematician : "taking mathematics," wrote Leib- 
nitz, "from the beginning of the world to the times 
when Newton lived, what he had done was much 
the better half." Newton was the greatest of astron- 
omers who, since the revival of learning, had ob- 
served the motions of the heavenly bodies and 
sought to find out why they moved. 

Copernicus, Tycho Brahe, Galileo, Kepler, New- 
ton, all are bound together as in a plot. Not one of 
them can be dissociated from the greatest of all 
discoveries. But Newton, the greatest of them all, 
revealed his greatness even more by saying: "If I 
have seen further than other men, it is because I 
have been standing on the shoulders of giants." 
Elsewhere he says : "All this was in the two plague 
years of 1665 and 1666 [he was then but twenty- 
four], for in those days I was in the prime of my 
age for invention, and minded mathematics and 
philosophy more than at any time since." All school 
children know these as the years of the plague and 
the fire; but very few, in school or out, connect 
these years with two other far-reaching events in 



ASTRONOMY A LIVING SCIENCE 15 

the world's history, the invention of the infinitesimal 
calculus and the discovery of the law of gravitation. 

We have passed over the name of Descartes, al- 
most contemporary with Galileo, the founder of 
modern dynamics, but his initiation of one of the 
greatest improvements of mathematical method 
cannot be overlooked. This era was the beginning 
of the Golden Age of Mathematics that embraced 
the lives of the versatile Euler, equally at home in 
dynamics and optics and the lunar theory; of La 
Grange, author of the elegant "Mecanique Ana- 
lytique"; and La Place, of the unparalleled "Meca- 
nique Celeste." With them and a fully elaborated 
calculus Newton's universal law had been extended 
to all the motions of the cosmos. Even the tides and 
precession of the equinoxes and Bradley's nutation 
were accounted for and explained. Mathematical or 
gravitational astronomy had attained its pinnacle — 
it seemed to be a finished science: all who were to 
come after must be but followers. 

The culmination of one great period, however, 
proved to be but the inception of another epoch in 
the development of the living science. 

The greatest observer of all time, with a tele- 
scope built by his own hands, had discovered a great 
planet far beyond the then confines of the solar sys- 
tem. Mathematicians would take care of Uranus, 
and Herschel was left free to build bigger telescopes 
still, and study the construction of the stellar uni- 
verse. Down to his day astronomy had dealt almost 
wholly with the positions and motions of the celes- 
tial bodies — astronomy was a science of where. 
To inquire what the heavenly bodies are, seemed 
to Herschel worthy of his keenest attention also. 
While "a knowledge of the construction of the 



16 ASTRONOMY TO-DAY 

heavens has always been the ultimate object of my 
observations/' as he said, and his ingenious method 
of star-gauging was the first practicable attempt to 
investigate the construction of the sidereal universe, 
he nevertheless devoted much time to the descrip- 
tion of nebulae and their nature, as well as their 
distribution in space. He was the founder of double- 
star astronomy, and his researches on the light of 
the stars by the simple method of sequences were 
the inception of the vast fields of stellar photometry 
and variable stars. The physics of the sun, also, was 
by no means neglected; and his lifework earned 
for him the title of father of descriptive astronomy. 

While progress and discovery in the earlier fields 
of astronomy were going on, the initial discoveries 
in the vast group of small planets were made at the 
beginning of the nineteenth century. The great 
Bessel added new life to the science by revolution- 
izing the methods and instruments of accurate 
observation, his work culminating in the measure 
of the distance of 61 Cygni, first of all the stars 
whose distance from the sun became known. 

Wonderful as was this achievement, however, a 
greater marvel still was announced just before the 
middle of the century — a new planet far beyond 
Uranus, whose discovery was made as a direct re- 
sult of mathematical researches by Adams and Le 
Verrier, and affording an extraordinary verification 
of the great Newtonian law. These were the days of 
great discoveries, and about this time the giant 
of all the astronomical tools of the century was 
erected by Lord Rosse, the "Leviathan" reflector 
with a speculum six feet in diameter, which re- 
mained for more than half a century the greatest 
telescope in the world, and whose epochal discovery 



ASTRONOMY A LIVING SCIENCE 17 

of spiral nebulae has greater significance than we 
yet know or perhaps even surmise. 

The living science was now at the height of a 
vigorous development, when a revolutionary dis- 
covery was announced by Kirchhoff which had been 
hanging fire nearly half a century — ^the half cen- 
tury, too, which had witnessed the invention of 
photography, the steam engine, the railroad, and the 
telegraph: three simple laws by which the dark 
absorption lines of a spectrum are interpreted, and 
the physical and chemical constitution of sun and 
stars ascertained, no matter what their distance 
from us. 

Huggins in England and Secchi in Italy were 
quick to apply the discovery to the stars, and Draper 
and Pickering by masterly organization have photo- 
graphed and classified the spectra of many hundred 
thousand stars of both hemispheres, a research of 
the highest importance which has proved of unique 
service in studies of stellar movements and the 
structure of the universe by Eddington and Shapley, 
Campbell and Kapteyn, with many others who are 
still engaged in pushing our knowledge far beyond 
the former confines of the universe. 

Few are the branches of astronomy that have not 
been modified by photograpiiy and the spectroscope. 
It has become a measuring tool of the first order of 
accuracy; measuring the speed of stars and nebulae 
toward and from us ; measuring the rotational speed 
of sun and planets, corona and Saturnian ring; 
measuring the distances of whole classes of stars 
from the solar system; measuring afresh even the 
distance of the sun — the yardstick of our immediate 
universe; measuring the drift of the sun with his 
entire family of planets twelve miles every second 



18 ASTRONOMY TO-DAY 

in the direction of Alpha Lyrae; and discovering 
and measuring the speed of binary suns too close 
together for our telescopes, and so making real 
the astronomy of the invisible. 

Impatient of the handicap of a turbulent atmos- 
phere, the living science has sought out mountain 
tops and there erected telescopes vastly greater than 
the "Leviathan" of a past century. There the sun 
in every detail of disk and spectrum is photographed 
by day, and stars with their spectra and the nebulae 
by night. Great streams of stars are discovered 
and the speed and direction of their drift ascer- 
tained. The marvels of the spiral nebulae are un- 
folded, their multitudinous forms portrayed and 
deciphered. 

And their distances ? And the distances of the still 
more wonderful clusters? Far, inconceivably far 
beyond the Milky Way. And are they "island uni- 
verses"? And can man, the measurer, measure the 
distance of the "mainland" beyond? 



CHAPTER II 

THE FIRST ASTRONOMERS 

WHO were the first astronomers? And who 
wrote the first treatise on astronomy, oldest 
of the sciences? 

Questions not easy to answer in our day. With 
the progress of archaeological research, or inquiry 
into the civilization and monuments of early 
peoples, it becomes certain that man has lived on 
this planet earth for tens of thousands of years in 
the past as an intelligent, observing, intellectual 
being; and it is impossible to assign any time so 
remote that he did not observe and philosophize 
upon the firmament above. 

We can hardly imagine a people so primitive that 
they would fail to regard the sun as "Lord of the 
Day," and therefore all important in the scheme of 
things terrestrial. Says Anne Bradstreet of the 
sun in her "Contemplations" : 

What glory's like to thee? 

Soul of this world, this universe's eye, 

No wonder some made thee deity. 

To the Babylonians belongs the credit of the 
oldest known work on astronomy. It was written 
nearly six thousand years ago, about B. c. 3800, by 
their monarch Sargon the First, King of Agade. 
Only the merest fragments of this historic treatise 
have survived, and they indicate the reverence of 

19 



20 ASTRONOMY TO-DAY 

the Babylonians for the sun. Another work by 
Sargon is entitled "Omens," which shows the inti- 
mate relationship of astronomy to mysticism and 
superstitious worship at this early date, and which 
persists even at the present day. 

As remotely as B. c. 3000, the sun-god Shamash 
and his wife Aa are carved upon the historic 
cylinders of hematite and lapis lazuli, and one of 
the oldest designs on these cylinders represents the 
sun-god coming out of the Door of Sunrise, while a 
porter is opening the Gate of the East. The 
Semitic religion had as its basis a reverence for the 
bodies of the sky; and Samson, Hebrew for sun, 
was probably the sun-god of the Hebrews. The 
Phoenician deity, Baal, was a sun-god under differ- 
ing designations ; and at the epoch of the Shepherd 
Kings, about B. C. 1500, during the Hyksos dynasty, 
the sun-god was represented by a circle or disk 
with extended rays ending in hands, possibly the 
precursor of the frequently recurring Egyptian 
design of the winged disk or winged solar globe. 
Hittites, Persians, and Assyrians, as well as the 
Phoenicians, frequently represented the sun-god 
in similar fashion in their sacred glyphs or 
carvings. 

For a long period in early human history, as- 
tronomy and astrology were pretty much the same. 
We can trace the history of astrology back as far 
as B. C. 3000 in ancient Babylonia. The motions of 
the sun, moon, and the five lucid planets of that 
time indicated the activity of the various gods 
who influenced human affairs. So the Babylonian 
priests devised an elaborate system of interpreting 
the phenomena of the heavens; and attaching the 
proper significance in human terms to everything 



THE FIRST ASTRONOMERS 21 

that took place in the sky. In Babylonia and As- 
syria it was the king and his people for whom the 
prognostications were made out. It was the same 
in Egypt. Later, about the fifth century B. c, 
astrology spread through Greece, where astrologers 
developed the idea of the influence of planets upon 
individual concerns. Astrology persisted through 
the Dark Ages, and the great astronomers Coper- 
nicus, Tycho, Kepler, Gassendi, and Huygens were 
all astrologers as well. Milton makes many refer- 
ences to planetary influence, our language has many 
words with a direct origin in astrology, and in our 
great cities to-day are many astrologers who pre- 
pare individual horoscopes of more than ordinary 
interest. 

It is difficult to assign the antiquity of the 
Chinese astronomy with any approach to definite- 
ness. Their earliest records appear to have been 
total eclipses of the sun, going back nearly 2,200 
years before the Christian era; and nearly a 
thousand years earlier the Hindu astronomy sets 
down a conjunction of all the planets, concerning 
which, however, there is doubt whether it was 
actually observed or merely calculated backward. 
Owing to a colossal misfortune, the burning of all 
native scientific books by order of the Emperor 
Tsin-Chi-Hwang-Ti, in B. c. 221, excepting only 
the volumes relating to agriculture, medicine, and 
astrology, the Chinese lost a precious mass of astro- 
nomical learning, accumulated through the ages. No 
less an authority than Wells Williams credits them 
with observing 600 solar eclipses between B. c. 2159 
and A. D. 1223, and there must have been some cen- 
turies of eclipses observed and recorded anterior to 
B. C. 2159, as this is the date assigned to the echpse 



22 ASTRONOMY TO-DAY 

which came unheralded by the astronomers royal, 
Hi and Ho, who had become intoxicated and forgot 
to -warn the Court, in accord with their duty. China 
was thereby exposed to the anger of the gods, and 
Hi and Ho were executed by his Majesty's com- 
mand. It is doubtful if there is an earlier record 
of any celestial phenomenon. 



CHAPTER III 

PYRAMID, TOMB, AND TEMPLE 

INQUIRY into the beginnings of astronomy in 
ancient Egypt reveals most interesting relations 
of the origins of the science to the life and work and 
worship of the people. Their astronomers were 
called the "mystery teachers of heaven"; their 
monuments indicate a civilization more or less ad- 
vanced ; and their temples were built on astronom- 
ical principles and dedicated to purpose of wor- 
ship. The Egyptian records carry us back many 
thousands of years, and we find that in Egypt, as 
in other early civilizations observation of the 
heavenly bodies may be embraced in three pretty 
distinct stages. Awe, fear^ wonder and worship 
were the first. Then came utility : a calendar was 
necessary to tell men when "to plow and sow, to 
reap and mow," and a calendar necessitated astro- 
nomical observations of some sort. Following this, 
the third direction required observations of celestial 
positions and phenomena also, because astrology, in 
which the potentates of every ancient realm believed, 
could only thrive as it was based on astronomy. 

Sun worship was preeminent in early Egypt as 
in India, where the primal antithesis between night 
and day struck terror in the unformed mind of man. 
In one of the Vedas occurs this significant song to 
the god of day: "Will the Sun rise again? Will our 
old friend the Dawn come back again? Will the 

23 



24 ASTRONOMY TO-DAY 

power of Darkness be conquered by the God of 
Light?" 

Quite different from India, however, is Egypt in 
matters of record: in India, records in papyrus, 
but no monuments of very great antiquity; in 
Egypt, no papyrus, but monuments of exceeding an- 
tiquity in abundance. Herodotus and Pliny have 
told us of the great antiquity of these monuments, 
even in their own day, and research by archaeologist 
and astronomer has made it certain that the pyra- 
mids were built by a race possessing great knowledge 
of astronomy. Their temples, too, were constructed 
in strict relation to stars. Not only are the tem- 
ples, as Edfu and Denderah, of exceeding interest 
in themselves, but associated with them are often 
huge monoliths of syenite, obelisks of many hundred 
tons in weight, which the astronomer recognizes as 
having served as observation pillars or gnomons. 
Specimens of these have wandered as far from 
home as Central Park and the bank of the Thames. 
But there is an even more remarkable wealth of 
temple inscriptions, zodiacs especially. 

Next to the sun himself was the worship of the 
Dawn and Sunrise, the great revelations of nature. 
There were numerous hymns to the still more 
numerous sun-gods and the powers of sunlight. 
Ra was the sun-god in his noontide strength ; Osiris, 
the dying sun of sunset. Only two gods were as- 
sociated with the moon, and for the stars a special 
goddess, Sesheta. Sacrifices were made at day- 
break; and the stars that heralded the dawn were 
the subjects of careful observation by the sacrificial 
priests, who must therefore have possessed a good 
knowledge of star places and names, doubtless in 
belts of stars extending clear around the heavens. 



PYRAMID, TOMB, AND TEMPLE 25 

These decans, as they were called, are the exact 
counterparts of the moon stations devised by the 
Arabians, Indians, and other peoples for a like 
purpose. 

The plane or circle of observation, both in Egypt 
and India, was always the horizon, whether the sun 
was observed or moon or stars. So the sun was 
often worshiped by the ancient Egyptians as the 
*^Lord of the Two Horizons." It is sometimes 
difficult to keep in mind the fact, in regard to all 
temples of the ancients, whether in Egypt or else- 
where, that in studying them we must deal with 
the risings or settings of the heavenly bodies in 
quite different fashion from that of the astrono- 
mer of to-day, who is mainly concerned only with 
observing them on the meridian. The axis of the 
temple shows by its direction the place of rising or 
setting: if the temple faces directly east or west, 
its amplitude is 0. Now the sun, moon, and planets 
are, as everyone knows, very erratic as to their 
amplitudes (i. e., horizon points) of rising and set- 
ting ; so it must have been the stars that engrossed 
the attention of the earliest builders of temples. 
After that, temples were directed to the rising sun, 
at the equinox or solstices. Then came the neces- 
sity of finding out about the inclination or obliquity 
of the ecliptic, and this is where the gnomon was 
em,ployed. 

At Karnak are many temples of the solstitial 
order: the wonderful temple of Amen-Ra is so or- 
iented that its axis stands in amplitude 26 degrees 
north of west, which is the exact amplitude of the 
sun at Thebes at sunset of the summer solstice. 
The axis of a lesser temple adjacent points to 26 
degrees south of east, which is the exact amplitude 



26 ASTRONOMY TO-DAY * 

of sunrise at the winter solstice. At Gizeh we find 
the temples oriented, not solstitially, but by the 
equinoxes, that is, they face due east and west. 
Peoples who worshiped the sun at the solstice must 
have begun their year at the solstice ; and Sir Nor- 
man Lockyer shows how the rise of the Nile, which 
took place at the summer solstice, dominated not 
only the industry but the astronomy and rehgion of 
Egypt. 

Looking into the question of temple orientation 
in other countries, as China, for example, Lockyer 
finds that the most important temple of that 
country, the Temple of the Sun at Peking, is oriented 
to the winter solstice; and Stonehenge, as has long 
been known, is oriented to sunrise at the summer 
solstice. 

In like fashion the rising and setting of many 
stars were utilized by the Egyptians, in both temple 
and pyramid ; and no astronomer who has ever seen 
these ancient structures and studied their orienta- 
tions can doubt that they were built by astronomers 
for use by astronomers of that day. The priests 
were the astronomers, and the temples had a deep 
religious significance, with a ceremony of exceeding 
magnificence wherever observations of heavenly 
bodies were undertaken, whether of sun or stars. 

Hindu and Persian astronomy must be passed 
over very briefly. Interesting as their systems are 
historically, there were few, if any, original contri- 
butions of importance, and the Indian treatises bear 
strong evidence of Greek origin. 



CHAPTER IV 

ORIGIN OF GREEK ASTRONOMY 

WHILE the Greeks laid the foundations of mod- 
ern scientific astronomy, they were not as a 
whole observers: rather philosophers, we should 
say. The later representatives of the Greek School, 
however, saw the necessity of observation as a basis 
of true induction; and they discovered that real 
progress was not possible unless their speculative 
ideas were sufficiently developed and made definite 
by the aid of geometry, so that they became capable 
of detailed comparison with observation. This was 
the necessary and ultimate test with them, and the 
same is true to-day. The early Greek philosophers 
were, however, mainly interested, not in observa- 
tions, but in guessing the causes of phenomena. 

Thales of Miletus, founder of the Ionian School, 
introduced the system of Egyptian astronomy into 
Greece, about the end of the seventh century B. c. 
He is universally known as the first astronomer who 
ever predicted a total eclipse of the sun that 
happened when he said it would : the eclipse of B. C. 
585. This he did by means of the Chaldean eclipse 
cycle of 18 years known as the Saros. 

Aristarchus of Samos was the first and most 
eminent of the Alexandrian astronomers, and his 
treatise "On the Magnitudes and Distances of the 
Sun and Moon" is still extant. This method of 
ascertaining how many times farther the sun is 

27 



28 ASTRONOMY TO-DAY 

than the moon is very simple, and geometrically- 
exact. Unfortunately it is impossible, even to-day, 
to observe with accuracy the precise time when the 
moon "quarters," (an observation essential to his 
method), because the moon's terminal, or line be- 
tween day and night, is not a straight line as re- 
quired by theory, but a jagged one. By his observa- 
tion, the sun was only twenty times farther away 
than the moon, a distance which we know to be 
nearly twenty times too small. 

His views regarding other astronomical questions 
were right, although they found little favor among 
contemporaries. Not only was the earth spherical, 
he said, but it rotated on its axis and also traveled 
round the sun. Aristarchus was, indeed, the true 
originator of the modern doctrine of motions in the 
solar system, and not Copernicus, seventeen cen- 
turies later; but Seleucus appears to have been his 
only follower in these very advanced conceptions. 
Aristarchus made out the apparent diameters of 
sun and moon as practically equal to one another, 
and inferred correctly that their real diameters are 
in proportion to their distances from the earth. 
Also he estimated, from observations during an 
eclipse of the moon, that the moon's diameter is 
about one-third that of the earth. Aristarchus 
appears to have been one of the clearest and most 
accurate thinkers among the ancient astronomers; 
even his views concerning the distances of the stars 
were in accord with the fact that they are immeasur- 
ably distant as compared with the distances of the 
sun, moon, and planets. 

Practically contemporary with Aristarchus were 
Timocharis and Aristillus, who were excellent ob- 
servers, and left records of position of sun and 



ORIGIN OF GREEK ASTRONOMY 29 

planets which were exceedingly useful to their suc- 
cessors, Hipparchus and Ptolemy in particular. 
Indeed their observations of star positions were 
such that, in a way, they deserve the fame of hav- 
ing made the first catalogue, rather than Hipparchus, 
to whom is universally accorded that honor. 

Spherical astronomy had its origin with the 
Alexandrian, school, many famous geometers, and in 
particular Euclid, pointing the way. Spherics, or 
the doctrine of the sphere, was the subject of nu- 
merous treatises, and the foundations were securely 
laid for that department of astronomical research 
which was absolutely essential to farther advance. 
The artisans of that day began to build rude mechan- 
ical adaptations of the geometric conceptions as 
concrete constructions in wood and metal, and it 
became the epoch of the origin of astrolabes and 
armillary spheres. 



CHAPTER V 

MEASURING THE EARTH- 
ERATOSTHENES 

ALL told, the Greek philosophers were probably 
-^ the keenest minds that ever inhabited the planet, 
and we cannot suppose them so stupid as to reject 
the doctrine of a spherical earth. In fact so certain 
were they that the earth's true figure is a sphere 
that Eratosthenes in the third century B. C. made 
the first measure of the dimensions of the terrestrial 
sphere by a method geometrically exact. 

At Syene in Upper Egypt the sun at the summer 
solstice was known to pass through the zenith at 
noon, whereas at Alexandria Eratosthenes esti- 
mated its distance as seven degrees from the zenith 
at the same time. This difference being about one- 
fiftieth of the entire circumference of a meridian. 
Eratosthenes correctly inferred that the distance 
between Alexandria and Syene must be one-fiftieth 
of the earth's circumference. So he measured the 
distance between the two and found it 5,000 stadia. 
This figured out the size of the earth with a per- 
centage of error surprisingly small when we con- 
sider the rough means with which Eratosthenes 
measured the sun's zenith distance and the distance 
between the two stations. 

Greatest of all the Greek astronomers and one 
of the greatest in the history of the science was 
Hipparchus who had an observatory at Rhodes in 

80 



MEASURING THE EARTH 31 

the middle of the second century B. c. His activi- 
ties covered every department of astronomy; he 
made extensive series of observations which he 
diligently compared with those handed down to 
him by the earlier astronomers, especially Aristillus 
and Timocharis. This enabled him to ascertain the 
motion of the equinoxial points, and his value of 
the constant of precession of the equinoxes is ex- 
ceedingly accurate for a first determination. 

In 134 B. c. a new star blazed out in the constella- 
tion Scorpio, and this set Hipparchus at work on a 
catalogue of the brighter stars of the firmament, a 
monumental work of true scientific conception, be- 
cause it would enable the astronomers of future 
generations to ascertain what changes, if any, were 
taking place in the stellar universe. There were 
1,080 stars in his catalogue, and he referred their 
positions to the ecliptic and the equinoxes. Also he 
originated the present system of stellar magnitudes 
or orders of brightness, and his catalogue was in 
use as a standard for many centuries. 

Hipparchus was a great mathematician as well, 
and he devoted himself to the improvement of the 
method of applying numerical calculations to geo- 
metrical figures: trigonometry, both plane and 
spherical, that is ; and by some authorities he is re- 
garded as the inventor of original methods in trigo- 
nometry. The system of spheres of Eudoxus did not 
satisfy him, so he devised a method of representing 
the paths of the heavenly bodies by perfectly uni- 
form motion in circles. There is slight evidence that 
Apollonius of Perga may have been the originator 
of the system, but it was reserved for Hipparchus 
to work it out in final form. This enabled him to 
ascertain the varying length of the seasons, and he 



32 ASTRONOMY TO-DAY 

fixed the true length of the year as 365% days. He 
had almost equal success in dealing with the ir- 
regularities of the moon's motion, although the 
problem is much more complicated. The distance 
and size of the moon, by the method of Aristarchus, 
were improved by him, and he worked out, for the 
distance of the sun, 1,200 radii of the earth— a 
classic for many centuries. 

Hipparchus devoted much attention to eclipses 
of both sun and moon, and we owe to him the first 
elucidation of the subject of parallax, or the effect 
of difference of position of an observer on the earth's 
surface as affecting the apparent projection of the 
moon against the sun when a solar eclipse takes 
place ; whereas an eclipse of the moon is unaffected 
by parallax and can be seen at the same time by 
observers everywhere, no matter what their loca- 
tion on the earth. Indeed, with all that Hipparchus 
achieved, we need not be surprised that astronomy 
was regarded as a finished science, and made prac- 
tically no progress whatever for centuries after his 
time. 

Then came Claudius Ptolemseus, generally known 
as Ptolemy, the last great name in Greek astronomy. 
He lived in Alexandria about the middle of the 
second century A. D. and wrote many minor as- 
tronomical and astrological treatises, also works on 
geography and optics, in the last of which the 
atmospheric refraction of rays of light from the 
heavenly bodies, apparently elevating them toward 
the zenith, is first dealt with in true form. 



Sci. Vol. 2—1 



CHAPTER VI 

PTOLEMY AND HIS GREAT BOOK 

PTOLEMY was an observer of the heavens, 
though not of the highest order; but he had 
all the work of his predecessors, best of all 
Hipparchus, to build upon. Ptolemy's greatest 
work was the *'Megale Syntaxis," generally known 
as the Almagest. It forms a nearly complete 
compendium of the ancient astronomy, and 
although it embodies much error, because built 
on a wrong theory, the Almagest nevertheless 
is competent to follow the motions of all the 
bodies in the sky with a close approach to ac- 
curacy, even at the present day. This marvel- 
ous work written at this critical epoch became as 
authoritative as the philosophy of Aristotle, and 
for many centuries it was the last word in the 
science. The old astrology held full sway, and 
the Ptolemaic theory of the universe supplied 
everything necessary: further progress, indeed, 
was deemed impossible. 

The Almagest comprises in all thirteen books, the 
first two of which deal with the simpler observations 
of the celestial sphere, its own motion and the ap- 
parent motions of sun, moon, and planets upon it. He 
discusses, too, the postulates of his system and ex- 
hibits great skill as an original geometer and mathe- 
matician. In the third book he takes up the length 
of the year, and in the fourth book similarly the 

33 Sci. Vol. 2—2 



34 ASTRONOMY TO-DAY 

moon and the length of the month. Here his mathe- 
matical powers are at their best, and he made a dis- 
covery of an inequality in the moon's motion known 
as the evection. Book five describes the construc- 
tion and use of the astrolabe, a combination of grad- 
uated circles with which Ptolemy made most of his 
observations. In the sixth book he follows mainly 
Hipparchus in dealing with eclipses of sun and 
moon. In the seventh and eighth books he discusses 
the motion of the equinox, and embodies a catalogue 
of 1,028 stars, substantially as in Hipparchus. The 
five remaining books of the Almagest deal with the 
planetary motions, and are the most important of all 
of Ptolemy's original contributions to astronomy. 
Ptolemy's fundamental doctrines were that the heav- 
ens are spherical in form, ail the heavenly motions 
being in circles. In his view, the earth too is spheri- 
cal, and it is located at the center of the universe, 
being only a points as it were, in comparison. All was 
founded on mere appearance combined with the phil- 
osophical notion that the circle being the only 
perfect curve, all motions of heavenly bodies must 
take place in earth-centered circles. For fourteen 
or fifteen centuries this false theory persisted, on 
the authority of Ptolemy and the Almagest, render- 
ing progress toward the development of the true 
theory impossible. 

Ptolemy correctly argued that the earth itself is a 
sphere that is curved from east to west, and from 
north to south as well, clinching his argument, as we 
do to-day, by the visibility of objects at sea, the 
lower portions of which are at first concealed from 
our view by the curved surface of the water which 
intervenes. To Ptolemy also the earth is at the 
center of the celestial sphere, and it has no motion 



PTOLEMY AND HIS GREAT BOOK 35 

of translation from that point; but his argument 
fails to prove this. Truth and error, indeed, are so 
deftly intermingled that one is led to wonder why 
the keen intelligence of this great philosopher per- 
mitted him to reject the simple doctrine of the 
earth's rotation on its axis. But if we reflect that 
there was then no science of natural philosophy or 
physics proper, and that the age was wholly unde- 
veloped along the lines of practical mechanics, we 
shall see why the astronomers of Ptolemy's time and 
subsequent centuries were content to accept the doc- 
trines of the heavens as formulated by him. 

When it came to explaining the movements of the 
"wandering stars," or planets, as we term them, the 
Ptolemaic theory was very happy in so far as ac- 
curacy was concerned, but very unhappy when it had 
to account for the actual mechanics of the cosmos in 
space. Sun and moon were the only bodies that went 
steadily onward, easterly: whereas all the others. 
Mercury, Venus, Mars, Jupiter, Saturn, although 
they moved easterly most of the time, nevertheless 
would at intervals slow down to stationary points, 
where for a time they did not move at all, and then 
actually go backward to the west, or retrograde, then 
become stationary again, finally resuming their regu- 
lar onward motion to the east. 

To help out of this difficulty, the worst possible 
mechanical scheme was invented, that known as the 
epicycle. Each of the five planets was supposed to 
have a fictitious "double," which traveled eastward 
with uniformity, attached to the end of a huge but 
mechanically impossible bar. The earth-centered 
circle in which this traveled round was called the 
"deferent." What this bar was made of, what 
stresses it would be subjected to, or what its size 



36 ASTRONOMY TO-DAY 

would have to be in order to keep from breaking — 
none of these questions seems to have agitated the 
ancient and medieval astronomers, any more than 
the flat-earth astronomy of the Hindu is troubled 
by the necessity of something to hold up the 
tortoise that holds up the elephant that holds up 
the earth. 

But at the end of this bar is jointed or swiveled 
another shorter bar^ to the revolving end of which is 
attached the actual planet itself ; and the second bar, 
by swinging once round the end of the primary ad- 
vancing bar, would account for the backward or ret- 
rograde motion of the planet as seen in the sky. For 
every new irregularity^ that was found, in the motion 
of Mars, for instance, a new and additional bar was 
requisitioned, until interplanetary space was hope- 
lessly filled with revolving bars, each producing one 
of the epicycles, some large, some small, that were 
needed to take up the vagaries of the several planets. 

The Arabic astronomers who kept the science alive 
through the Middle Ages added epicycle to epicycle, 
until there was every justification for Milton's verses 
descriptive of the sphere: 

With Centric and Eccentric scribbled o'er. 
Cycle and Epicycle, Orb in Orb. 



CHAPTER VII 

ASTRONOMY OF THE MIDDLE AGES 

WITH the fall of Alexandria and the victory of 
Mohammed throughout the West, and a conse- 
quent decline in learning, supremacy in science 
passed to the East and centered round the caliphs 
of Bagdad in the seventh and eighth centuries. 
They were interested in astronomy only as a prac- 
tical, and to them useful, science, in adjusting the 
complicated lunar calendar of the Mohammedans, in 
ascertaining the true direction of Mecca which 
every Mohammedan must know, and in the revival 
of astrology, to which the Greeks had not attached 
any particular significance. 

Harun-al-Rashid ordered the Almagest and 
many other Greek works translated, of which the 
modern world would otherwise no doubt never have 
heard, as the Greek originals are not extant. 

Splendid observatories were built at Damascus 
and Bagdad, and fine instruments patterned after 
Greek models were continuously used in observing. 
The Arab astronomers, although they had no clocks, 
were nevertheless so fully impressed with the im- 
portance of time that they added extreme value to 
their observations of eclipses, for example, by setting 
down the altitudes of sun or stars at the same time. 
On very important occasions the records were certi- 
fied on oath by a body of barristers and astrono- 
mers conjointly— a precedent which fortunately has 
never been followed. 

37 



38 ASTRONOMY TO-DAY 

About the middle of the ninth century, the Caliph 
Al-Mamun directed his astronomers to revise the 
Greek measures of the earth's dimensions, and they 
had less reverence for the Almagest than existed in 
later centuries: indeed, Tabit ben Korra invented 
and applied to the tables of the Almagest a theoreti- 
cal fluctuation in the position of the ecliptic which 
he called "trepidation," which brought sad confu- 
sion into astronomical tables for many succeeding 
centuries. 

• Albategnius was another Arab prince whose 
record in astronomy in the ninth and tenth centuries 
was perhaps the best: the Ptolemaic values of the 
precession of the equinoxes and of the obliquity of 
the ecliptic were improved by new observations, and 
his excellence as mathematician enabled him to make 
permanent improvements in the astronomical ap- 
plication of trigonometry. 

Abul Wefa was the last of the Bagdad astrono- 
mers in the latter half of the tenth century, and his 
great treatise on astronomy known as the Alma- 
gest is sometimes confused with Ptolemy's work. 
Following him was Ibn Yunos of Cairo, whose 
labors culminated in the famous Hakemite Tables, 
which became the standard in mathematical and 
astronomical computations for several centuries. 

Mohammedan astronomy thrived, too, in Spain 
and northern Africa. Arzachel of Toledo published 
the Toledan Tables, and his pupils made improve- 
ments in instruments and the methods of calcula- 
tion. The Giralda was built by the Moors in Seville 
in 1196, the first astronomical observatory on the 
continent of Europe; but within the next half cen- 
tury both Seville and Cordova became Christian 
again, and Arab astronomy was at an end. 



ASTRONOMY OF THE MIDDLE AGES 39 

Through many centuries, however, the science had 
been kept aHve, even if no great original advances 
had been achieved; and Arab activities have modi- 
fied our language very materially, adding many such 
words as almanac, zenith, and radii, and a wealth of 
star names, as Aldebaran, Rigel, Betelgeuse, Vega, 
and so on. 

Meanwhile, other schools of astronomy had de- 
veloped in the East, one at Meraga near the modern 
Persia, where Nassir Eddin, the astronomer of 
Hulagu Khan, grandson of the Mongol emperor 
Genghis Khan, built and used large and carefully 
constructed instruments^ translated all the Greek 
treatises on astronomy, and published a laborious 
work knowm as the Ilkhanic Tables, based on the 
Hakemite Tables of Ibn Yunos. 

More important still was the Tartar school of as- 
tronomy under Ulugh Begh, a grandson of Tamer- 
lane, who built an observatory at Samarcand in 1420, 
published new tables of the planets, and made with 
his excellent instruments the observations for a new 
catalogue of stars, the first since Hipparchus, the 
star places being recorded with great precision. 

The European astronomy of the Middle Ages 
amounted to very little besides translation from the 
Arabic authors into Latin, with commentaries. As- 
tronomers under the patronage of Alfonso X of Leon 
and Castile published in 1252 the Alfonsine Tables, 
which superseded the Toledan tables and were ac- 
cepted everywhere throughout Europe. Alfonso 
published also the "Libros del Saber," perhaps the 
first of all astronomical cyclopedias, in which is said 
to occur the earliest diagram representing a planet- 
ary orbit as an ellipse: Mercury's supposed path 
round the earth as a center. 



40 ASTRONOMY TO-DAY 

Purbach of Vienna about the middle of the 15th 
century began his "Epitome of Astronomy" based 
on the "Almagest" of Ptolemy, which was finished by 
his collaborator Regiomontanus, who was an expert 
in mathematics and published a treatise on trigo- 
nometry with the first table of sines calculated for 
every minute from 0° to 90°, a most helpful contribu- 
tion to theoretical astronomy. 

Regiomontanus had a very picturesque career, 
finally taking up his residence in Nuremberg, where 
a wealthy citizen named Walther became his patron, 
pupil, and collaborator. The artisans of the city 
were set at work on astronomical instruments of the 
greatest accuracy, and the comet of 1472 was the 
first to be observed and studied in true scientific 
fashion. Regiomontanus was very progressive and 
the invention of the new art of 'printing gave him an 
opportunity to publish Purbach's treatise, which 
went through several editions and doubtless had 
much to do in promoting dissatisfaction with the 
ancient Ptolemaic system, and was thus most sig- 
nificant in preparing a background for the coming 
of the new Copernican order. 

The Nuremberg presses popularized astronomy 
in other important ways, issuing almanacs, the first 
precursors of our astronomical Ephemerides. Re- 
giomontanus was practical as well, and invented a 
new method of getting a ship's position at sea, with 
tables so accurate that they superseded all others 
in the great voyages of discovery, and it is probable 
that they were employed by Columbus in his dis- 
covery of the American continent. Regiomontanus 
had died several years earlier, in 1475 at Rome, where 
he had gone by invitation of the Pope to effect a re- 
formation in the calendar. He was only forty, and 



ASTRONOMY OF THE MIDDLE AGES 41 

his patron Walther kept on with excellent observa- 
tions, the first probably to be corrected for the effect 
of atmospheric refraction, although its influence had 
been known since Ptolemy. The Nuremberg School 
lasted for nearly two centuries. 

Nearly contemporary with Regiomontanus were 
Fracastoro and Peter Apian, whose original observa- 
tions on comets are worthy of mention because they 
first noticed that the tails of these bodies always 
point away from the sun. Leonardo da Vinci was 
the first to give the true explanation of earth-shine 
on the moon, and similarly the moon-illumination 
of the earth ; and this no doubt had great weight in 
disposing of the popular notion of an essential dif- 
ference of nature between the earth and celestial 
bodies — all of which helped to prepare the way for 
Copernicus and the great revolution in astronomical 
thought. 



CHAPTER VIII 

COPERNICUS AND THE NEW ERA 

THROUGHOUT the Middle Ages the progress of 
astronomy was held back by a combination of 
untoward circumstances. A prolonged reaction from 
the heights attained by the Greek philosophers was 
to be expected. The uprising of the Mohammedan 
world, and the savage conquerors in the East did not 
produce conditions favorable to the origin and devel- 
opment of great ideas. 

At the birth of Copernicus, however, in 1473, the 
time was ripening for fundamental changes from 
the ancient system, the error of which had helped to 
hold back the development of the science for cen- 
turies. The fifteenth century was most fruitful in 
a general quickening of intelligence, the invention of 
printing had much to do with this, as it spread a 
knowledge of the Greek writers, and led to conflict 
of authorities. Even Aristotle and Ptolemy were 
not entirely in harmony, yet each was held inviolate. 
It was the age of the Reformation, too, and near the 
end of the century the discovery of America exerted 
a powerful stimulus in the advance of thought. 

Copernicus searched the works of the ancient 
writers and philosophers, and embodied in this new 
order such of their ideas as commended themselves 
in the elaboration of his own system. 

Pythagoras alone and his philosophy looked in the 
true direction. Many believe that he taught that the 
sun, not the earth, is at the center of our solar sys- 

42 



COPERNICUS AND THE NEW ERA 43 

tern ; but his views were mingled with the specula- 
tive philosophy of the Greeks, and none of his writ- 
ings, barring a few meager fragments, have come 
down to our modern age. 

To many philosophers, through all these long cen- 
turies, the true theory of the celestial motions must 
have been obvious, but their viev/s were not formu- 
lated, nor have they been preserved in writing. So 
the fact remains that Copernicus alone first proved 
the truth of the system which is recognized to-day. 
This he did in his great treatise entitled "De Revolu- 
tionibus Orbium Coelestium," the first printed copy 
of which was dramatically delivered to him on his 
deathbed, in May, 1543. The seventy years of his 
life were largely devoted to the preparation of this 
work, which necessitated many observations as 
well as intricate calculations based upon them. 
Being ,a canon in the church, he naturally hesi- 
tated about publishing his revolutionary views, his 
friend Rheticus first doing this for him in outline 
in 1540. 

So simple are the great principles that they may 
be embodied in very few words; what appears to 
us as the daily revolution of the heavens is not a 
real motion, but only an apparent one; that is, the 
heavens are at rest, while the earth itself is in 
motion, turning round an axis which passes through 
its center. And the second proposition is that the 
earth is simply one of the six known planets; and 
they all revolve round the sun as the true center. 
The solar system, therefore, is "heliocentric," or 
sun-centered, not "geocentric" or earth-centered, as 
taught by the Ptolemaic theory. 

Copernicus demonstrates clearly how his system 
explains the retrograde motion of the planets and 



44 ASTRONOMY TO-DAY 

their stationary points, no matter whether they are 
within the orbit of the earth, as Mercury and Venus, 
or outside of it, as Mars, Jupiter, and Saturn. His 
system provides also the means of ascertaining with 
accuracy the proportions of the solar system, or 
the relative distances of the planets from the sun 
and from each other. In this respect also his sys- 
tem possessed a vast advantage over that of 
Ptolemy, and the planetary distances which Coper- 
nicus computed are very close approximations to 
the measures of the present day. 

Reinhold revised the calculations of Copernicus 
and prepared the "Tabulae Prutenicse," based on the 
"De Revolutionibus," which proved far superior to 
the Alfonsine Tables, ,and were only supplanted 
by the Rudolphine Tables of Kepler. On the whole 
we may regard the lifework of Copernicus as 
fundamentally the most significant in the history 
and progress of astronomy. 



CHAPTER IX 

TYCHO, THE GREAT OBSERVER 

CLEAR as Copernicus had made the demonstration 
of the truth of his new system, it nevertheless 
failed of immediate and universal acceptance. The 
Ptolemaic system was too strongly intrenched, and 
the motions of all the bodies in the sky were too 
well represented by it. Accurate observations were 
greatly needed, and the Landgrave William IV. of 
Hesse built the Cassel Observatory, which made a 
new catalogue of stars, and introduced the use of 
clocks to carry on the time as measured by the uni- 
form motion of the celestial sphere. Three years 
after the death of Copernicus, Tycho Brahe was 
born, and when he was 30 the King of Denmark 
built for him the famous observatory of Uraniborg, 
where the great astronomer passed nearly a quarter 
of a century in critically observing the positions of 
the stars and planets. Tycho was celebrated as a 
designer and constructor of new types of astro- 
nomical instruments, and he printed a large volume 
of these designs, which form the basis of many in 
use at the present day. Unfortunately for the 
genius of Tycho and the significance of his work, 
the invention of the telescope had not yet been 
made, so that his observations had not the modern 
degree of accuracy. Nevertheless, they were des- 
tined to play a most important part in the progress 
of astronomy. 

45 



46 ASTRONOMY TO-DAY 

Tycho was sadly in error in his rejection of th^ 
Copernican system, although his reasons, in his day, 
seemed unanswerable. If the outer planets were 
displaced among the stars by the annual motion of 
the earth round the sun, he argued, then the fixed 
stars must be similarly displaced — unless indeed 
they be at such vast distances that their motions 
would be too slight to be visible. Of course we know 
now that this is really true, and that no instruments 
that Tycho was able to build could possibly have 
detected the motions, the effects of which we now 
recognize in the case of the nearer fixed stars in 
their annual, or parallactic, orbits. 

The remarkably accurate instruments devised by 
Tycho Brahe and employed by him in improving 
the observations of the positions of the heavenly 
bodies were no doubt built after descriptions of 
astrolabes such as Hipparchus used, as described 
by Ptolemy. In his "Astronomise Instaurata Meca- 
nica" we find illustrations and descriptions of many 
of them. 

One is a polar astrolabe, mounted somewhat as 
a modern equatorial telescope is, and the meridian 
circle is adjustable so that it can be used in any 
place, no matter what its latitude might be. There 
is a graduated equatorial ring at right angles to 
the polar axis, so that the astrolabe could be used 
for making observations outside the meridian as 
well as on it. This equatorial circle slides through 
grooves, and is furnished with movable sights, and 
a plumb line from the zenith or highest point of 
the meridian circle makes it possible to give the 
necessary adjustment in the vertical. Screws for 
adjustment at the bottom are provided, just as in 
our modern instruments, and two observers were 



TYCHO, THE GREAT OBSERVER 47 

necessary, taking their sights simultaneously; un- 
less, as in one type of the instrument, a clock, or 
some sort of measure of time, was employed. 

Another early type of instrument is called by 
Tycho the ecliptic astrolabe {Armillse Zodiacales, 
or the Zodiacal Rings) . It resembles the equatorial 
astrolabe somewhat, but has a second ring inclined 
to the equatorial one at an angle equal to the 
obliquity of the ecliptic. In observing, the equa- 
torial ring was revolved round till the ecliptic ring 
came into coincidence with the plane of the ecliptic 
in the sky. Then the observation of a star's longi- 
tude and latitude, as referred to the ecliptic plane, 
could be made, quite as well as that of right ascen- 
sion and declination on the equatorial plane. But it 
was necessary to work quickly, as the adjustment on 
the ecliptic would soon disappear and have to be 
renewed. 

Tycho is often called the father of the science of 
astronomical observation, because of the improve- 
ments in design and construction of the instruments 
he used. His largest instrument was a mural quad- 
rant, a quarter-circle of copper, turning parallel 
to the north-and-south face of a wall, its axis turn- 
ing on a bearing fixed in the wall The radius of 
this quadrant was nine feet, and it was graduated 
or divided so as to read the very sm-all angle of ten 
seconds of arc— an extraordinary degree of pre- 
cision for his day. 

Tycho built also a very large alt=azimuth quad- 
rant, of six feet radius. Its operation was very 
much as if his mural quadrant could be swung 
round in azimuth. At several of the great observa- 
tories of the present day, as Greenwich and Wash- 
ington, there are instruments of a similar type. 



48 ASTRONOMY TO-DAY 

but much more accurate, because the mechanical 
work in brass and steel is executed by tools that are 
essentially perfect, and besides this the power of 
the telescope is superadded to give absolute direc- 
tion, or pointing on the object under observation. 

Excellent clocks are necessary for precise ob- 
servation with such an instrument; but neither 
Tycho Brahe, nor Hevelius was provided with such 
accessories. Hevelius did not avail himself of the 
telescope as an aid to precision of observation, 
claiming that pinhole sights gave him more accu- 
rate results. It was a dispute concerning this ques- 
tion that Halley was sent over from London to 
Danzig to arbitrate. 

There could be but one way to decide; the tele- 
scope with its added power magnifies any displace- 
ment of the instrument, and thereby enables the 
observer to point his instrument more exactly. So 
he can detect smaller errors and differences of 
direction than he can without it. And what is of 
great importance in more modern astronomy, the 
telescope makes it possible to observe accurately the 
position of objects so faint that they are wholly 
invisible to the naked eye. 



CHAPTER X 

KEPLER, THE GREAT CALCULATOR 

MOST fortunate it was for the later development 
of astronomical theory that Tycho Brahe not 
only was a practical or observational astronomer of 
the highest order, but that he confined himself studi- 
ously for years to observations of the places of the 
planets. Of Mars he accumulated an especially long 
and accurate series, and among those who assisted 
him in his v/ork was a young and brilliant pupil 
named Johann Kepler. 

Strongly impressed with the truth of the Coperni- 
can System, Kepler was free to reject the erroneous 
compromise system devised by Tycho Brahe, and 
soon after Tycho's death Kepler addressed himself 
seriously to the great problem that no one had ever 
attempted to solve, viz : to find out what the laws of 
motion of the planets round the sun really are. Of 
course he took the fullest advantage of all that 
Ptolemy and Copernicus had done before him, and 
he had in addition the splendid observations of 
Tycho Brahe as a basis to work upon. 

Copernicus, while he had effected the tremendous 
advance of substituting the sun for the earth as the 
center of motion, nevertheless clung to the errone- 
ous notion of Ptolemy that all the bodies of the sky 
must perforce move at uniform speeds, and in cir- 
cular curves, the circle being the only ''perfect 
curve." Kepler was not long in finding out that 

49 



50 ASTRONOMY TO-DAY 

this could not be so, and he found it out because 
Tycho Brahe's observations were much more ac- 
curate than any that Copernicus had employed. 

Naturally he attempted the nearest planet first, 
and that was Mars — ^the planet that Tycho had 
assigned to him for research. How fortunate that 
the orbit of Mars was the one, of all the planets, to 
show practically the greatest divergence from the 
ancient conditions of uniform motion in a perfectly 
circular orbit! Had the orbit of Mars chanced to 
be as nearly circular as is that of Venus, Kepler 
might well have been driven to abandon his search 
for the true curve of planetary motion. 

However, the facts of the cosmos were on his 
side, but the calculations essential in testing his 
various hypotheses were of the most tedious nature, 
because logarithms were not yet known in his day. 
His first discovery was that the orbit of Mars is 
certainly not a circle, but oval or elliptic in figure. 
And the sun, he soon found, could not be in the 
center of the ellipse, so he made a series of trial 
calculations with the sun located in one of the 
foci of the ellipse instead. 

Then he found he could make his calculated places 
of Mars agree quite perfectly v/ith Tycho Brahe's 
observed positions, if only he gave up the other 
ancient requisite of perfectly uniform motion. On 
doing this, it soon appeared that Mars, when in 
perihelion^ or nearest the sun, always moved 
swiftest, v/hile at its greatest distance from the 
sun, or aphelion, its orbital velocity was slowest. 

Kepler did not busy himself to inquire why these 
revolutionary discoveries of his were as they were ; 
he simply went on making enough trials on Mars, 
and then on the other planets in turn, to satisfy 



KEPLER, THE GREAT CALCULATOR 51 

himself that all the planetary orbits are elliptical, 
not circular in form, and are so located in space 
that the center of the sun is at one of the two foci 
of each orbit. This is known as Kepler's first law 
of planetary motion. 

The second one did not come quite so easy; it 
concerned the variable speed v/ith which the planet 
moves at every point of the orbit. We must remem- 
ber how handicapped he was in solving this prob- 
lem : only the geometry of Euclid to work with, and 
none of the refinements of the higher mathematics 
of a later day. But he finally found a very simple 
relation which represented the velocity of the planet 
everywhere in its orbit. It was this : if we calculate 
the area sv/ept, or passed over, by the planet's 
radius vector (that is, the line joining its center to 
the sun's center) during a week's time near peri- 
helion, and then calculate the similar area for a 
week near aphelion, or indeed for a week when 
Mars is in any intermediate part of its orbit, we 
shall find that these areas are all equal to each 
other. So Kepler formulated his second great law of 
planetary motion very simply: the radius vector of 
any planet describes, or sweeps over, equal areas in 
equal times. And he found this was true for all the 
planets. 

But the real genius of the great mathematician 
was shown in the discovery of his third law, which 
is more complex and even more significant than 
the other two — a law connecting the distances of 
the planets from the sun with their periods of revo- 
lution about the sun. This cost Kepler many addi- 
tional years of close calculation, and the resulting 
law, his third law of planetary motion is this : The 
cubes of the mean or average distances of the 



52 ASTRONOMY TO-DAY 

planets from the sun are proportional to the squares 
of their times of revolution around him. 

So Kepler had not only disposed of the sacred 
theories of motion of the planets held by the 
ancients as inviolable, but he had demonstrated the 
truth of a great law which bound ail the bodies of 
the solar system together. So accurately and com- 
pletely did these three laws account for all the mo- 
tions, that the science of astronomy seemed as if 
finished; and no matter how far in the future a 
time might be assigned, Kepler's laws provided 
the means of calculating the planet's position for 
that epoch as accurately as it would be possible 
to observe it. Kepler paused here, and he died 
in 1630. 



CHAPTER XI 

GALILEO, THE GREAT EXPERIMENTER 

THE fifteenth and sixteenth centuries, containing 
the lives and work of Copernicus, Tycho, Galileo, 
Kepler, Huygens, Halley, and Newton, were a veri- 
table Golden Age of astronomy. All these men were 
truly great and original investigators. 

None had a career more picturesque and popular 
than did Galileo. Born a few years earlier and 
dying a few years later than Kepler, the work of 
each of these two great astronomers was wholly in- 
dependent of the other and in entirely different 
fields. Kepler was discovering the laws of planetary 
motion, while Galileo was laying the secure founda- 
tions of the new science of dynamics, in particular 
the laws of falling bodies, that was necessary before 
Kepler's laws could be fully understood. When only 
eighteen Galileo's keen power of observation led to 
his discovery of the laws of pendulum motion, sug- 
gested by the oscillation to and fro of a lamp in the 
cathedral of Pisa. 

The world-famous leaning tower of this place, 
where he was born, served as a physical laboratory 
from the top of which he dropped various objects, 
and thus was led to formulate the laws of falling 
bodies. He proved that Aristotle was all wrong in 
saying that a heavy body must fall swifter in pro- 
portion to its weight than a lighter one. These and 
other discoveries rendered him unpopular with his 
associates, who christened him the "Wrangler." 

53 



54 ASTRONOMY TO-DAY 

The new system of Copernicus appealed to him; 
and when he, first of all men, turned a telescope on 
the heavenly bodies, there was Venus with phases 
like those of the moon, and Jupiter with satellites 
traveling about it — a Copernican system in min- 
iature. Nothing could have happened that would 
have provided a better demonstration of the truth 
of the new system and the falsity of the old. His 
marvelous discoveries caused the greatest excite- 
ment — consternation even, among the anti-Copemi- 
cans. Galileo published the "Sidereus Nuncius,'' with 
many observations and drawings of the moon, 
which he showed to be a body not wholly dissimilar 
to the earth: this, too, was obviously of great mo- 
ment in corroboration of the Copernican order and 
in contradiction to the Ptolemaic, which maintained 
sharp lines of demarcation between things terres- 
trial and things celestial. 

His telescopes, small as they were, revealed to him 
anomalous appearances on both sides of the planet 
Saturn which he called ans^, or handles. But their 
subsequent disappearance was unaccountable to 
him, and later observers, who kept on guessing 
ineffectively till Huygens, nearly a half century 
after, showed that the true nature of the appendage 
was a ring. Spots on the sun were frequently ob- 
served by Galileo and led to bitter controversies. 
He proved, however, that they were objects on the 
sun itself, not outside it, and by noticing their re- 
peated transits across the sun's disk, he showed that 
the sun turned round on his axis in a little less than 
a month — another analogy to the like motion of the 
earth on the Copernican plan. 

Galileo's appointment in 1610 as "First Philoso- 
pher and Mathematician" to the Grand Duke of 



GALILEO, THE GREAT EXPERIMENTER 55 

Tuscany gave him abundant time for the pursuit 
of original investigations and the preparation of 
books and pamphlets. His first visit to Rome the 
year following was the occasion of a reception with 
great honor by many cardinals and others of high 
rank. His lack of sympathy with others whose 
views differed from his, and his naturally contro- 
versial spirit, had begun to lead him headlong into 
controversies with the Jesuits and the church, which 
culminated in his censure by the authorities of the 
church and persecution by the Inquisition. 

In 1618 three comets appeared, and Galileo was 
again in controversial hot water with the Jesuits. 
But it led to the publication five years later of "II 
Saggiatore" (The Assayer), of no great scientific 
value, but only a brilliant bit of controversial litera- 
ture dedicated to the newly elevated Pope, Urban 
VIII. Later he wrote through several years a great 
treatise, more or less controversial in character, 
entitled a "Dialogue on the Two Chief Systems of 
the World" between three speakers, and extending 
through four successive days. Simplicio argues for 
the Aristotelians, Salviati for the Copernicans, 
while Sagredo does his best to be neutral. It will 
always be a very readable book, and we are for- 
tunate to have a recent translation by Professor 
Crew of Evanston. 

Here we find the first suggestion of the 
modern method of getting stellar parallaxes, the 
relative parallax, that is, of two stars in the same 
field — a method not put into service till Bessel's 
time, two centuries later. But the most important 
chapters of the "Dialogue" deal with Galileo's inves- 
tigations of the laws of motion of bodies in general, 
which he applied to the problem of the earth's 



56 ASTRONOMY TO-DAY 

motion. In this he really anticipated Newton in the 
first of his three laws of motion, and in a subse- 
quent work, dealing with the theory of projectiles, 
he reaches substantially the results of Newton's 
second law of motion, although he gave no general 
statement of the principle. Nevertheless, in the 
epoch where his life was lived and his work done, 
his telescopic discoveries, combined with his dyna- 
mic researches in untrodden fields, resulted in the 
complete and final overthrow of the ancient system 
of error, and the secure establishment of the Co- 
pernican system beyond further question and dis- 
cussion. Only then could the science of astronomy 
proceed unhampered to the fullest development by 
the master minds of succeeding centuries. 



CHAPTER XII 

AFTER THE GREAT MASTERS 

FOLLOWING Kepler and Galileo was a half cen- 
tury of great astronomical progress along many 
lines laid out by the work of the great masters. The 
telescope seemed only a toy, but its improvement in 
size and quality showed almost inconceivable pos- 
sibilities of celestial discoveries. 

Hevelius of Danzig took up the study of the moon, 
and his "Selenographia" was finely illustrated by 
plates which he not only drew but engraved himself. 
Lunar names of mountains, plains, and craters we 
owe very largely to him. Also he published among 
other works two on comets, the second of which was 
published in 1668 and called the "Cometographia," 
the first detailed account of all the comets observed 
and recorded to date. 

Many were the telescopes turned on the planet 
Saturn, and every variety of guess was made as to 
the actual shape and physical nature of the weird 
appendages discovered by Galileo. The true solution 
was finally reached by Huygens, whose mechanical 
genius had enabled him to grind and polish larger 
and better lenses than his contemporaries ; in 1659 
he pubhshed the "Systema Saturnium" interpreting 
the ring and the cause of its various configurations, 
and the first discovery of a Saturnian satellite is 
due to him. 

Gascoigne in England about 1640 was the first 
to make the important application of the microm- 

57 



58 ASTRONOMY TO-DAY 

eter to enhance the accuracy of measurement of 
/small angles in the telescopic field; an invention 
made and applied independently many years later 
by Huygens in Holland and Auzout and Picard in 
France, where the instrument was first regularly 
employed as an accessory in the work of an 
observatory. 

Another Englishman, Jeremiah Horrocks, was 
the first observer of a transit of Venus over the 
disk of the sun, in 1639. Horrocks was possessed 
of great ability in calculational astronomy also. 
This was about the time of the invention of the 
pendulum clock by Huygens, which in conjunction 
with the later invention of the transit instrument 
by Roemer wrought a revolution in the exacting 
art of practical astronomy. This was because it 
enabled the time to be carried along continuously, 
and the revolution of the earth could be utilized 
in making precise measures of the position of 
sun, moon, and stars. Louis XIV had just founded 
the new Observatory at Paris in 1668, and Picard 
was the first to establish regular time-observations 
there. 

Huygens followed up the motion of the pendulum 
in theory as well as practice in his "Oscillatorium 
Horologium" (1673), showing the way to measure 
the force of gravity, and his study of circular 
motion showed the fundamental necessity of some 
force directed toward the center in planetary 
motions. 

The doctrine of the sphericity of the earth being 
no longer in doubt, the great advance in accuracy of 
astronomical observation indicated to Willebrord 
Snell in Holland the best way to measure art arc 
of meridian by triangulation. Picard repeated the 



AFTER THE GREAT MASTERS 59 

measurements near Paris with even greater ac- 
curacy, and his results were of the utmost sig- 
nificance to Newton in establishing his law of 
gravitation. 

Domenico Cassini, an industrious observer, 
voluminous writer, and a strong personality, devised 
telescopes of great size, discovered four Saturnian 
satellites and the main division in the ring of 
Saturn, determined the rotation periods of Mars 
and Jupiter, and prepared tables of the eclipses of 
Jupiter's satellites. At his suggestion Richer under- 
took an expedition to Cayenne in latitude 5 degrees 
north, where it was found that the intensity of 
gravity was less than at Paris, and his clock there- 
fore lost time, thus indicating that the earth was 
not a perfect sphere as had been thought, but a 
spheroid instead. 

The planet Mars passed a near opposition, and 
Richer's observations of it from Cayenne, when 
combined with those of Cassini and others in 
France, gave a new value of the sun's parallax and 
distance, really the first actual measurement worth 
the name in the history of astronomy. 

To close this era of signal advance in astronomy 
we may cite a discovery by Roemer of the first 
order : no less than that of the velocity of transmis- 
sion of light through space. At the instigation of 
Picard, Roemer in studying the motions of Jupiter's 
satellites found that the intervals between eclipses 
grew less and less as Jupiter and the earth ap- 
proached each other, and greater and greater than 
the average as the two planets separated farther 
and farther. Roemer correctly attributed this dif- 
ference to the progres^sive motion of light and a 
rough value of its velocity was calculated, though 



60 ASTRONOMY TO-DAY 

not accepted oy astronomers generally for more 
than a century. 

Why^ the laws of Kepler should be true, Kepler 
himself was unable to say. Nor could anyone else 
in that day answer these questions: (1) The planets 
move in orbits that are elliptical not circular — ^why 
should they move in an impeifect curve, rather than 
the perfect one in which it had always been taught 
that they moved? (2) Why should our planet vary 
its velocity at all, and travel now fast, now slow; 
especially why should the speed so vary that the 
line of varying length, joining the planet to the sun, 
always passes over areas proportional to the time 
of describing them? And (3) Why should there be 
any definite relation of the distances of planets from 
the sun to their times of revolution about him? 
Why should it be exactly as the cube of one to the 
square of the other? 

We must remember that the Copernican sys- 
tem itself was not yet, in the beginning of the 
seventeenth century, accepted universally; and 
the great minds of that period were most con- 
cerned in overturning the erroneous theory of 
Ptolemy. 

The next step in logical order was to find a basic 
explanation of the planetary motions, and Des- 
cartes and his theory of vortices are worthy of men- 
tion, among many unsuccessful attempts in this 
direction. Descartes was a brilliant French philos- 
opher and mathematician, but his hypothesis of a 
multitude of whirlpools in the ether, while ingenious 
in theory, was too vague and indefinite to account for 
the planetary motions with anj^ approach to the 
precision with which the laws of Kepler represented 
them. 



I 



AFTER THE GREAT MASTERS 61 

Another great astronomer whose labors helped 
immensely in preparing the way for the signal dis- 
coveries that were soon to come was Huygens, a 
man of versatility as natural philosopher, mechani- 
cian, and astronomical observer. Huygens was born 
thirteen years before the death of Galileo, and to 
the discovery of the laws of motion by the latter 
Huygens added researches on the laws of action of 
centrifugal forces. Neither of them, however, ap- 
peared to see the immediate bearing on the great 
general problem of celestial motions in its true light, 
and it was reserved for another generation, and an 
astronomer of another country, to make the one 
fundamental discovery that should explain the 
whole by a single simple law. 



CHAPTER XIII 



NEWTON AND MOTION 

''TTOW is it that you are able to make these great 
-*--■- discoveries?" was once asked of Sir Isaac New- 
ton, facile princeps of all philosophers, and the dis- 
coverer of the great law of universal gravitation. 

"By perpetually thinking about them," was New- 
ton's terse and illuminating reply. He had set for 
himself the definite problem of Kepler's laws : why 
is it that they are true, and is there not some single, 
general law that will embody all the circumstances 
of the planetary motions? 

Newton was born in 1643, the year after the 
death of Galileo. He had a thorough training in 
the mathematics of his day, and addressed himself 
first to an investigation and definite formulation of 
the general laws of motion, which he found to be 
three in number, and which he was able to put in 
very simple terms. The first one is: Any body, 
once it is set in motion, will continue to move for- 
ward in a straight line with a uniform velocity for- 
ever, provided it is acted upon by no force what- 
ever. In other words, a state of motion is as natural 
as a state of rest (rest in relation to things every- 
where adjacent) in which we find all things in 
general. 

Here on earth where gravity itself pulls all ob- 
jects downward toward the earth, and where re- 
sistance of the air tends to hold a moving body 

62 



NEWTON AND MOTION 63 

back and bring it to rest, and where friction from 
contact with whatever material substance may be 
in its path is perpetually tending to neutralize all 
motion — with all three of these forces always at 
work to stop a moving body, the truth of this first 
and fundamental law of motion was not apparent on 
the surface. 

Till Galileo's time everyone had made the mis- 
take of supposing that some force or other must 
be acting continually on every moving body to keep 
it in motion. Ptolemy, Copernicus, Kepler, Leo- 
nardo da Vinci — all failed to see the truth of this 
law which Newton developed in the immortal 
Principia. And at the present day it is not always 
easy to accept at first, although the progress of 
mechanical science, by reducing friction and resist- 
ance, has produced machines in v/hich motion of 
large masses may be kept up indefinitely with the 
application of only the merest minimum of force. 
Once a planet is set in motion round the sun, 
it would go on forever through frictionless, non- 
resistant space; but there must be a central force, 
as Huygens saw clearly, to hold it in its orbit. 
Otherwise it would at any moment take the direc- 
tion of a tangent to the orbit. Here is where New- 
ton's second law of motion comes in, and he formu- 
Ilated it with great definiteness. When any force 
acts on a moving body, its deviation from a straight 
line will be in the direction of the force applied and 
proportional to that force. 
In accord with this law, NevTton first began to 
inquire whether the force of attraction here on 
earth, which everyone commonly recognizes as 
gravity, drawing all things down toward the center 



64 ASTRONOMY TO-DAY 

It is found in operation on the summits of mountain 
peaks, and the clouds above them and the rain 
falling from them are obviously drawn downv^ard 
by the same force. May it not extend outward into 
space, even as far as the moon? 

This was an audacious question, but Nev.i:on not 
only asked, but tried to answer it in the year 1665, 
when he was only twenty-three. On the surface of 
the earth this attraction is strong enough to draw 
a falling body downward through a vertical space 
of sixteen feet in a second of time. What ought it 
to be at the distance of the moon. The distance of 
the moon in Newton's time was better known in 
terms of the earth's size than was the size of the 
earth itself : the earth's radius was known to be one- 
sixtieth of the moon's distance, but the earth's 
diameter was thought to be something undeF 7,000 
miles, so that Newton's first calculations were most 
disappointing, and he laid them aside for nearly 
twenty years. 

Meanwhile the French astronomers led by Picard 
had measured the earth anew, and showed it to be 
nearly 8,000 miles in diameter. As soon as Newton 
learned of this, he revised his calculations, and 
found that by the law of the inverse square the 
moon, in one second, should fall away from a tan- 
gent to its orbit one thirty-six hundredth of sixteen 
feet. 

This accorded exactly with his original supposi- 
tion that the earth's attraction extended to the 
moon. So he concluded that the force which makes a 
stone fall, or an apple, as the story goes, is the same 
force that holds the moon in its orbit, and that this 
force diminishes in the exact proportion that the 
square of the distance from the earth's center in- 



NEWTON AND MOTION 65 

creases. The moon, indeed, becomes a falling body ; 
only, as Kingdon Clifford puts it: "She is going so 
fast and is so far off that she falls quite around to 
the other side of the earth, instead of hitting it; 
and so goes on forever." 

Newton goes on in the Principia to explain the 
extension of gravitation to the other bodies of the 
solar system beyond the earth and moon. Clearly 
the same gravitation that holds the moon in its 
orbit round the earth, must extend outward from 
the sun also, and hold all the planets in their orbits 
centered about him. Newton demonstrates by cal- 
culation based on Kepler's third law that (1) the 
forces drawing the planets toward the sun are in- 
versely as the squares of their mean distances from 
him; and (2) if the force be constantly directed 
toward the sun, the radius vector in an elliptic orbit 
must pass over equal areas in equal times. 



Sci. Vol. 2—3 



CHAPTER XIV 

NEWTON AND GRAVITATION 

SO all of Kepler's laws could be embodied in a 
single law of gravitation toward a central body, 
whose force of attraction decreases outward in 
exact proportion as the square of the distance in- 
creases. 

Only one farther step had to be taken, and this 
the most complicated of all: he must make all the 
bodies of the sky conform to his third law of motion. 
This is: Action and reaction are equal, or the 
mutual actions of any two bodies are always equal 
and oppositely directed. There must be mutual at- 
tractions everywhere: earth for sun as well as sun 
for earth, moon for sun and sun for moon, earth for 
Venus and Venus for earth, Jupiter for Saturn and 
Saturn for Jupiter, and so on. 

The motions of the planets in the undisturbed 
ellipses of Kepler must be impossible. As observa- 
tions of the planets became more accurate, it was 
found that they really did fail to move in exact 
accord with Kepler's laws unmodified. Newton 
was unable, with the imperfect processes of the 
mathematics of his day to ascertain whether the de- 
viations then known could be accounted for by his 
law of gravitation; but he nevertheless formulated 
the law with entire precision, as follows: 

Every particle of matter in the universe attracts 
every other particle with a force exactly propor- 

6Q 



NEWTON AND GRAVITATION 67 

tioned to the product of their masses, and inversely 
as the square of the distance between their centers. 

The centuries of astronomical research since 
Newton's day, however, have verified the great law 
with the utmost exactness. Practically every ir- 
regularity of lunar and planetary motion is ac- 
counted for; indeed, the intricacies of the problems 
involved, and the nicety of their solution, have led 
to the invention of new mathematical processes ade- 
quate to the difficulties encountered. 

And about the middle of the last century, when 
Uranus departed from the path laid out for it by 
the mathematical astronomers, its orbital devia- 
tions were made the basis of an investigation which 
soon led to the assignment of the position where a 
great planet could be found that would account for 
the unexplained irregularities of the motion of 
Uranus. And the immediate discovery of this 
planet, Neptune, became the most striking verifica- 
tion of the Newtonian law that the solar system 
could possibly afford. 

The astronomers of still later days investigating 
the statelier motions of stellar systems find the 
Newtonian law regnant everywhere among the 
stars where our most powerful telescopes have as 
yet reached. So that Newton's law is known as the 
law of Universal Gravitation, and its author is 
everywhere held as the greatest scientist of the 
ages. 

Newton's Principia may be regarded as the cul- 
minating research of the inductive method, and 
further outline of its contents is desirable. It is 
divided into three books following certain intro- 
ductory sections. The first book treats of the prob- 
lems of moving bodies, the solutions being worked 



68 ASTRONOMY TO-DAY 

out generally and not with special reference to 
astronomy. The second book deals with the motion 
of bodies through resistant media, as fluids, and 
has very little significance in astronomy. The third 
book is the all important one, and ap^plies his gen- 
eral principles to the case of the actual solar system, 
providing a full explanation of the motions of all 
the bodies of the system known in his day. Any- 
one who critically reads the Principia of Newton 
will be forced to conclude that its author was a 
genius in the highest sense of the word. The ele- 
gance and thoroughness of the demonstrations, and 
the completeness of application of the law of gravi- 
tation are especially impressive. 

The universality of his new law was the feature 
to which he gave particular attention. It was clear 
to him that the gravitation of a planet, although 
it acted as if wholly concentrated at the center, 
was nevertheless resident in every one of the par- 
ticles of which the planet is composed. Indeed, his 
universal law was so formulated as to make every 
particle attract every other particle ; and an investi- 
gation known as the Cavendish experiment — a re- 
search of great delicacy of manipulation — not only 
proves this, but leads also to a measurement of 
the earth's mean density, from, which we can cal- 
culate approximately how much the earth actually 
weighs. 

Another way to attack the same problem is by 
measuring the attraction of mountains, as Maske- 
lyne, Astronomer Royal of Scotland did on Mount 
Schehallien in Scotland, which was selected because 
of its sheer isolation. The attraction of the moun- 
tain deflected the plumb-lines by measurable 
amounts, the volume of the mountain was care- 



NEWTON AND GRAVITATION 69 

fully ascertained by surveys, and geologists found 
out what rocks composed it. So the weight of the 
entire mountain became pretty well known, and 
combining this with the observed deflection, an in- 
dependent value of the earth's weight was found. 

Still other methods have been applied to this 
question, and as an average it is found that the 
materials composing the earth are about five and 
a half times as heavy as water, and the total 
weight of the earth is something like six sex- 
tillions of tons. 

What is the true shape of the earth? And does 
the earth's turning round on its axis affect this 
shape? Newton saw the answer to these questions 
in his law of gravitation. A spherical figure fol- 
lowed as a matter of course from the mutual 
attraction of all materials composing the earth, pro- 
viding it was at rest, or did not turn round on its 
axis. But rotation bulges it at the equator and 
draws it in at the poles, by an amount which 
calculation shows to be in exact agreement with 
the amount ascertained by actual measurement of 
the earth itself. 

Another curious effect, not at first apparent, was 
that all bodies carried from high latitudes toward 
the equator would get lighter and lighter, in conse- 
quence of the centrifugal force of rotation. This 
was unexpectedly demonstrated by Richer when 
the French Academy sent him south to observe 
Mars in 1672. His clock had been regulated exactly 
in Paris, and he soon found that it lost time when 
set up at Cayenne. The amount of loss was found by 
observation, and it was exactly equal to the calcu- 
lated effect that the reduction of gravity by cen- 
trifugal action should prodiice. 



70 ASTRONOMY TO-DAY 

Also Nev/ton saw that his law of gravitation 
would afford an explanation of the rise and fall of 
the tides. The water on the side of the earth toward 
the moon, being nearer to the moon, would be more 
strongly attracted toward it, and therefore raised 
in a tide. And the water on the farther side of the 
earth av/ay from the moon, being at a greater dis- 
tance than the earth itself, the moon would attract 
the earth more strongly than this mass of water, 
tending therefore to draw the earth away from the 
v/ater, and so raising at the same time a high tide 
on the side of the earth away from the moon. As 
the earth turns round on its axis, therefore, two 
tidal waves continually follow each other at inter- 
vals of about twelve hours. 

The sun, too, joins its gravitating force with that 
of the moon, raising tides nearly half as high as 
those which the moon produces, because the sun's 
vaster mass makes up in large part for its much 
greater distance. At first and third quarters of 
the moon, the sun acts against the moon, and the 
difference of their tide-producing forces gives us 
"neap tides" ; while at new moon and full, sun and 
moon act together, and produce the maximum effect 
known as "spring tides." 

Newton passed on to explain, by the action of 
gravitation also, the precession of the equinoxes, 
a phenomenon of the sky discovered by Hipparchus, 
who pretty well ascertained its amount, although 
no reason for it had ever been assigned. The plane 
of the earth's equator extended to the celestial 
sphere marks out the celestial equator, and the two 
opposite points where it intersects the plane of the 
ecliptic, or the earth's path round the sun, are 
called the equinoctial points, or simply the equi- 



NEWTON AND GRAVITATION 71 

noxes. And precession of the equinoxes is the motion 
of these points westward or backward, about 50 
seconds each year, so that a complete revolution 
round the ecliptic would take place in about 26,000 
years. 

Newton saw clearly how to explain this: it is 
simply due to the attraction of the sun's gravitation 
upon the protuberant bulge around the earth's 
equator, acting in conjunction with the earth's rota- 
tion on its axis, the effect being very similar to that 
often seen in a spinning top, or in a gyroscope. The 
moon moving near the ecliptic produces a preces- 
sional effect, as also do the planets to a very slight 
degree; and the observed value of precession is the 
same as that calculated from gravitation, to a high 
degree of precision. 

Newton died in 1727, too early to have witnessed 
that complete and triumphant verification of his 
law which ultimately has accounted for practically 
every inequality in the planetary motions caused by 
their mutual attractions. The problems involved are 
far beyond the complexity of those which the mathe- 
matical astronomer has to deal with, and the mathe- 
maticians of France deserve the highest credit for 
improving the processes of their science so that 
obstacles which appeared insuperable were one 
after another overcome. 

Newton's method of dealing with these problems 
was mainly geometric, and the insufficiency of this 
method was apparent. Only when the French 
mathematicians began to apply the higher methods 
of algebra was progress toward the ultimate goal 
assured. D'Alembert and Clairaut for a time were 
foremost in these researches, but their places were 
soon taken by Lagrange, who wrote the "Mecanique 



72 ASTRONOMY TO-DAY 

Analytique," and Laplace, whose "Mecanique 
Celeste" is the most celebrated work of all. In large 
part these works are the basis of the researches of 
subsequent mathematical astronomers who, strictly- 
speaking, cannot as yet be said to have arrived at 
a complete and rigorous solution of all the problems 
which the mutual attractions of all the bodies of 
the solar system have originated. 

It may well be that even the mathematics of the 
present day are incompetent to this purpose. When 
the brilliant genius of Sir William Hamilton in- 
vented quaternion analysis and showed the marvel- 
ous facility with which it solved the intricate prob- 
lems of physics, there was the expectation that its 
application to the higher problems of mathematical 
astronomy might effect still greater advances; but 
nothing in that direction has so far eventuated. 
Some astronomers look for the invention of new 
functions with numerical tables bearing perhaps 
somewhat the relation to present tables of log- 
arithms, sines, tangents, and so on, that these tables 
do to the simple multiplication table of Pythagoras. 



CHAPTER XY 

AFTER NEWTOH 

WE have said that practically all the motions in 
the solar system have been accounted for by the 
Newtonian law of gravitation. It will be of in- 
terest 'to inquire into the instances that lead to 
qualification of this absolute statement. 

One relates to the planet Mercury, whose orbit 
or path round the sun is the most elliptical of all 
the planetary orbits. This will be explained a little 
later. 

The moon has given the mathematical astron- 
omers more trouble than any other of the celestial 
bodies, for one reason because it is nearest to us 
and very minute deviations in its motion are there- 
fore detectible. Halley it was who ascertained two 
centuries ago that the moon's motion round the 
earth was not uniform, but subject to a slight accel- 
eration which greatly puzzled Lagrange and La- 
place, because they had proved exactly this sort of 
thing to be impossible, unless indeed the body in 
question should be acted on by some other force 
than gravitation. But Laplace finally traced the 
cause to the secular or very slow reduction in the 
eccentricity of the earth's ovv^n orbit. The sun's 
action on the moon was indeed progressively chang- 
ing from century to century in such manner as to 
accelerate the moon's own motion in its orbit round 
the earth. 

73 



74 ASTRONOMY TO-DAY 

Adams, the eminent English astronomer, revised 
the calculations of Laplace, and found the effect in 
question only half as great as Laplace had done; 
and for years a great mathematical battle was on 
between the greatest of astronomical experts in this 
field of research. Adams, in conjunction with De- 
launay, the greatest of the French mathematicians 
a half century ago, won the battle in so far as the 
mathematical calculations were concerned; but the 
moon continues to the present day her slight and 
perplexing deviation, as if perhaps our standard 
time-keeper, the earth, by its rotation round its axis, 
were itself subject to variation. Although many in- 
vestigations have been made of the uniformity of 
the earth's rotation, no such irregularity has been 
detected, and this unexplained variation of the 
moon's motion is one of the unsolved problems of 
the gravitational astronomer of to-day. 

But we are passing over the most impressive of 
all the earlier researches of Lagrange and Laplace, 
which concerned the exceedingly slow changes, 
technically called the secular variations of* the ele- 
ments of the planetary orbits. These elements are 
geometrical relations which indicate the form of 
the orbit, the size of the orbit, and its position 
in space; and it was found that none of these re- 
lations or quantities are constant in amount or 
direction, but that all, with but one exception, 
are subject to very slow, or secular, change, 
or oscillation. 

This question assumed an alarming significance 
at an early day, particularly as it affected the eccen- 
tricity of the earth's orbit round the sun. Should 
it be possible for this element to go on increasing 
for indefinite ages, clearly the earth's orbit would 



AFTER NEWTON 75 

become more and more elliptical, and the sun would 
come nearer and nearer at perihelion, and the earth 
would drift farther and farther from the sun at 
aphelion, until the extremes of temperature would 
bring all forms of life on the earth to an end. The 
refined and powerful analysis of Lagrange, how- 
ever, soon allayed the fears of humanity by account- 
ing for these slow progressive changes as merely 
part of the regular system of mere oscillations, in 
entire accord with the operation of the law of gravi- 
tation; and extending throughout the entire plane- 
tary system. Indeed, the periods of these oscilla- 
tions were so vast that none of them were shorter 
than 50,000 years, while they ranged up to two 
million years in length — "great clocks of eternity 
which beat ages as ours beat seconds." 

About a century ago, an eminent lecturer on 
astronomy told his audience that the problem of 
weighing the planets might readily be one that 
would seem wholly impossible to solve. To measure 
their sizes and distances might well be done, but 
actually to ascertain how many tons they weigh — ? 
never ! 

Yet if a planet is fortunate enough to have one 
satellite or more, the astronomer's method of weigh- 
ing the planet is exceedingly simple; and all the 
major planets have satellites except the two in- 
terior ones, Mercury and Venus. As the satellite 
travels round its primary, just as the moon does 
round the earth, two elements of its orbit need to 
be ascertained, and only two. First, the mean dis- 
tance of the satellite from its primary, and second 
the time of revolution round it. 

Now it is simply a case of applying Kepler's 
third law. First take the cube of the satellite's dis- 



76 ASTRONOMY TO-DAY 

tance and divide it by the square of the time of 
revolution. Similarly take the cube of the planet's 
distance from the sun and divide by the square of 
the planet's time of revolution round him. The 
proportion, then, of the first quotient to the second 
shows the relation of the mass (that is the weight) 
of the planet to that of the sun. In the case of 
Jupiter, we should find it to be 1,050, in that of 
Saturn 3,500, and so on. 

The range of planetary masses, in fact, is very 
curious, and is doubtless of much significance in 
the cosmogony, with which we deal later. If we 
consider the sun and his eight planets, the mass or 
weight of each of the nine bodies far exceeds the 
combined mass of all the others which are lighter 
than itself. 

To illustrate: suppose we take as our unit of 
weight the one-billionth part of the sun's weight; 
then the planets in the order of their masses will be 
Mercury, Mars, Venus, Earth, Uranus, Neptune, 
Saturn, and Jupiter. According to their relative 
masses, then. Mercury being a five-millionth part 
the weight of the sun will be represented by 200; 
similarly Venus, a four hundred and twenty-five 
thousandth part by 2,350, and so on. Then we have 

Mercury 200 

Mars 340 



Sum of weights of Mercury and Mars 540 

Venus 2,350 



Sum of weights of Mercury, Mars, 
and Venus 2,890 

The Earth 3,060 



Sum of weights of four inner planets 5,950 



AFTER NEWTON 77 

Uranus 44,250 

Sum of weights of five planets 50,200 

Neptune 51,600 

Sum of weights of six planets 101,800 

Saturn 285,580 

Sum of weights of seven planets. . . . 387,380 

Jupiter 954,300 

Sum of weights of all the planets. . 1,341,680 

Mass or weight of the sun 1,000,000,000 

Curious and interesting it is that Saturn is 
nearly three times as heavy as the six lighter 
planets taken together, Jupiter between two and 
three times heavier than all the other planets com- 
bined, while the sun's mass is 750 times that of all 
the great planets of his system rolled into one. 

All the foregoing masses, except those of Mer- 
cury and Venus, are pretty accurately known be- 
cause they were found by the satellite method just 
indicated. Mercury's mass is found by its disturb- 
ing effects on Encke's comet whenever it approaches 
very near. The mass of Venus is ascertained by 
the perturbations in the orbital motion of the 
earth. In such cases the Newtonian law of gravita- 
tion forms the basis of the intricate and tedious 
calculations necessary to find out the mass by this 
indirect method. 

Its inferiority to the satellite method was strik- 
ingly shown at the Observatory in Washington soon 
after the satellites of Mars were discovered in 1877. 
The inaccurate mass of that planet, as previously 
known by months of computation based upon years 
and years of observation, was immediately dis- 



78 ASTRONOMY TO-DAY 

carded in favor of the new mass derived from the 
distance and period of the outer satellite by only 
a few minutes' calculation. 

In weighing" the planets^ astronomers always use 
the sun as the unit. What then is the sun's own 
weight? Obviously the law of gravitation answers 
this question, if we compare the sun's attraction 
with the earth's at equal distances. First we con- 
ceive of the sun's mass as if all compressed into 
a globe the size of the earth, and calculate how far 
a body at the surface of this globe would fall in one 
second. The relation of this number to 16.1 feet, the 
distance a body falls in one second on the actual 
earth, is about 330,000, which is therefore the num- 
ber of times the sun's weight exceeds that of the 
earth. 

A word may be added regarding the force of 
gravitation and what it really is. As a matter of 
fact Newton did not concern himself in the least 
with this inquiry, and says so very definitely. What 
he did was to discover the law according to which 
gravitation acts everywhere throughout the solar 
system. And although many physicists have en- 
deavored to find out what gravitation really is, its 
cause is not yet known. In some manner as yet 
mysterious it acts instantaneously over distances 
great and small alike, and no substance has been 
found which, if we interpose it between two bodies, 
has in any degree the effect of interrupting their 
gravitational tendency toward each other. 

While the Newtonian law of gravitation has been 
accepted as true because it explained and accounted 
for all the motions of the heavenly bodies, even in- 
cluding such motions of the stars as have been sub- 
jected to observation, astronomers have for a long 



AFTER NEWTON 79 

time recognized that quite possibly the law might 
not be absolutely exact in a mathematical sense, 
and that deviations from it would surely make their 
appearance in time, 

A crude instance of this was suggested about a 
century ago, when the planet Uranus was found to 
be deviating from the path m^arked out for it by 
Bouvard's tables based on the Newtonian law ; and 
the theory was advocated by many astronomers 
that this law, while operant at the medium dis- 
tances from the sun where the planets within 
Jupiter and Saturn travel, could not be expected to 
hold absolutely true at the vast distance of Uranus 
and beyond. The discovery of Neptune in 1846, 
however, put an end to all such speculation, and has 
universally been regarded as an extraordinary 
verification of the law, as indeed it is. 

When, however, Le Verrier investigated the orbit 
of Mercury he found an excess of motion in the 
perihelion point of the planet's orbit which neither 
he nor subsequent investigators have been able to 
account for by Newtonian gravitation, pure and 
simple. If Newton's theory is absolutely true, the 
excess motion of Mercury's perihelion remains a 
mystery. 

Only one theory has been advanced to account for 
this discrepancy, and that is the Einstein theory 
of gravitation. This ingenious speculation was first 
propounded in comprehensive form nearly fifteen 
years ago, and its author has developed from it 
mathematical f ormulse which appear to yield results 
even more precise than those based on the New- 
tonian theory. 

In expressing the difference between the law of 
gravitation and his own conception, Einstein says : 



80 ASTRONOMY TO-DAY 

"Imagine the earth removed, and in its place sus- 
pended a box as big as a moon or a whole house and 
inside a man naturally floating in the center, there 
being no force whatever pulling him. Imagine, 
further, this box being, by a rope or other con- 
trivance, suddenly jerked to one side, which is 
scientifically termed 'difform motion/ as opposed 
to 'uniform motion/ The person would then natur- 
ally reach bottom on the opposite side. The re- 
sult would consequently be the same as if he 
obeyed Newton^s law of gravitation, while, in fact, 
there is no gravitation exerted whatever, which 
proves that difform motion will in every case pro* 
duce the same effects as gravitation .... The term 
relativity refers to time and space. According to 
Galileo and Newton, time and space were absolute 
entities, and the moving systems of the universe 
were dependent on this absolute time and space. On 
this conception was built the science of mechanics. 
The resulting formulas sufficed for all motions of 
a slow nature; it was found, however, that they 
would not conform to the rapid motions apparent 
in electrodynamics .... Briefly the theory of special 
relativity discards absolute time and space, and 
makes them in every instance relative to moving 
systems. By this theory all phenomena in electro- 
dynamics, as well as mechanics, hitherto irreducible 
by the old formulae, were satisfactorily explained." 
Natural phenomena, then, involving gravitation 
and inertia, as in the planetary motions, and 
electro-magnetic phenomena, including the motion 
of light, are to be regarded as interrelated, and not 
independent of one another. And the Einstein 
theory would appear to have received a striking 
verification in both these fields. On this theory the 



AFTER NEWTON 81 

Newtonian dynamics fails when the velocities con- 
cerned are a near approach to that of light. The 
Newtonian theory, then, is not to be considered as 
wrong, but in the light of a first approximation. 
Applying the new theory to the case of the motion 
of Mercury's perihelion, it is found to account for 
the excess quite exactly. 

On the electro-magnetic side, including also the 
motion of light, a total eclipse of the sun affords 
an especially favorable occasion for applying the 
critical test, whether a huge mass like the sun would 
or would not deflect toward itself the rays of hght 
from stars passing close to the edge of its disk, or 
limb. A total eclipse of exceptional duration oc- 
curred on May 29, 1919, and the two eclipse parties 
sent out by the Royal Society of London and the 
Royal Astronomical Society were equipped espe- 
cially with apparatus for making this test. Their 
stations were one on the east coast of Brazil and the 
other on the west coast of Africa. 

Accurate calculation beforehand showed just 
where the sun would be among the stars at the time 
of the eclipse ; so that star plates of this region were 
taken in England before the expeditions went out. 
Then, during the total eclipse, the same regions were 
photographed with the eclipsed sun and the corona 
projected against them. To make doubly sure, the 
stars were a third time photographed some weeks 
after the eclipse, when the sun had moved away 
from that particular region. 

Measuring up the three sets of plates, it was 
found that an appreciable deflection of the light of 
the stars nearest alongside the sun actually exists ; 
and the amount of it is such as to afford a fair 
though not absolutely exact verification of the 



82 ASTRONOMY TO-DAY 

theory. The observed deflection may of course be 
due to other causes, but the Enghsh astronomers 
generally regard the near verification as a triumph 
for the Einstein theory. Astronomers are already 
beginning preparations for a repetition of the 
eclipse programme with all possible refinement of 
observation, v^hen the next total eclipse of the sun 
occurs, September 20, 1922, visible in Australia and 
the islands of the Indian Ocean. 

A third test of the theory is perhaps more critical 
than either of the others, and this necessitates a 
displacement of spectral lines in a gravitational 
field toward the red end of the spectrum; but the 
experts who have so far made measures for de- 
tecting such displacement disagree as to its actual 
existence. The work of St. John at Mt. Wilson is 
unfavorable to the theory, as is that of Evershed 
of Kodiakanal, who has made repeated tests on the 
spectrum of Venus, as well as in the cyanogen 
bands of the sun. 

The enthusiastic advocates of the Einstein theory 
hold that, as Newton proved the three laws of 
Kepler to be special cases of his general law, so 
the "universal relativity theory'' will enable eventu- 
ally the Newtonian law to be deduced from the Ein- 
stein theory. "This is the way we go on in science, 
as in everything else," wrote Sir George Airy, 
Astronomer Royal ; "we have to make out that some- 
thing is true; then we find out under certain cir^ 
cumstances that it is not quite true; and then we 
have to consider and find out how the departure can 
be explained." Meanwhile, the prudent person 
keeps the open mind. 



CHAPTER XVI 

HALLEY AND HIS COMET 

H ALLEY is one of the most picturesque charac- 
ters in all astronomical history. Next to Newton 
himself he was most intimately concerned in giving 
the Nev/tonian law to the world. 

Edmund Halley was born (1656) in stirring 
times. Charles I. had just been executed, and it 
was the era of CromwelFs Lord Protectorate and 
the wars with Spain and Holland. Then followed 
(1660) the promising but profligate Charles IL 
(who nevertheless founded at Greenwich the great- 
est of all observatories when Halley was nineteen), 
the frightful ravages of the Black Plague, the 
tyrannies of James II., and the Revolution of 1688 
— all in the early manhood of Halley, whose scien- 
tific life and works marched with much of the vigor 
of the contending personalities of state. 

The telescope had been invented a half century 
earlier, and Galileo's discoveries of Jupiter's moons 
and the phases of Venus had firmly established the 
sun-centered theory of Copernicus, 

The sun's distance, though, was known but 
crudely ; and why the stars seemed to have no yearly 
orbits of their own corresponding to that of the 
earth was a puzzle. Newton was well advanced 
toward his supreme discovery of the law of uni- 
versal gravitation; and the authority of Kepler 
taught that comets travel helter-skelter through 



84 ASTRONOMY TO-DAY 

space in straight lines past the earth, a perpetual 
menace to humanity. 

"Ugly monsters," that comets always were to the 
ancient world, the medieval church perpetuated this 
misconception so vigorously that even now these 
harmless, gauzy visitors from interstellar space 
possess a certain "wizard hold upon our imagina- 
tion." This entertaining phase of the subject is 
excellently treated in President Andrew D. White's 
"History of the Doctrine of Comets," in the Papers 
of the American Historical Association. Halley's 
brilliant comet at its earlier apparitions had been 
no exception. 

Halley's father was a wealthy London soap 
maker, who took great pride in the growing intel- 
lectuality of his son. Graduating at Queen's College, 
Oxford, the latter began his astronomical labors at 
twenty by publishing a work on planetary orbits; 
and the next year he voyaged to St. Helena to cata- 
logue the stars of the southern firmament, -to 
measure the force of terrestrial gravity, and observe 
a transit of Mercury over the disk of the sun. 

While clouds seriously interfered with his obser- 
vations on that lonely isle, what he saw of the 
transit led to his invention of "Halley's method," 
which, as applied to the transit of Venus, though 
not till long after his death, helped greatly in the 
accurate determination of the sun's distance from 
the earth. Halley's researches on the proper motions 
of the stars of both hemispheres soon made him fa- 
mous, and it was said of him, "If any star gets dis- 
placed on the globe, Halley will presently find it out." 

His return to London and election to the Royal 
Society (of which he was many years secretary) 
added much to his fame, and he was commissioned 



HALLEY AND HIS COMET 85 

by the society to visit Danzig and arbitrate an 
astronomical controversy between Hooke and He- 
velius, both his seniors by a generation. 

On the continent he associated with other great 
astronomers, especially Cassini, who had already 
found three Saturnian moons; and it was then he 
observed the great comet of 1680, which led up to 
the most famous event of Halley's life. 

The seerlike Seneca may almost be said to have 
predicted the advent of Halley, when he wrote 
("Quaestiones Naturales," vii) : "Some day there 
will arise a man who will demonstrate in what region 
of the heavens comets pursue their way; why they 
travel apart from the planets ; and what their sizes 
and constitution are. Then posterity will be amazed 
that simple things of this sort were not explained 
before." 

To Newton it appeared probable that cometary 
voyagers through space might have orbits of their 
own; and he proved that the comet of 1680 never 
swerved from such a path. As it could nowhere 
approach within the moon's orbit, clearly threats 
of its wrecking the earth and punishing its inhab- 
itants ought to frighten no more. 

Halley then became intensely interested in 
comets, and gathered whatever data concerning the 
paths of all these bodies he could find. His first 
great discovery was that the comets seen in 1531 by 
Apian, and in 1607 by Kepler, traveled round the 
sun in identical paths with one he had himself 
observed in 1682. A still earlier appearance of 
Halley's comet (1456) seems to have given rise to 
a popular and long-reiterated myth of a papal bull 
excommunicating "the Devil, the Turk^ and the 
Comet." 



86 ASTRONOMY TO-DAY 

No longer room for doubt : so certain was Halley 
that all three were one and the same comet, com- 
pleting the round of its orbit in about seventy-six 
years, that he fearlessly predicted that it would be 
seen again in 1758 or 1759. And with equal con- 
fidence he might have foretold its return in 1835 
and 1910; for all three predictions have come true 
to the letter. 

Halley's span of existence did not permit his 
living to see even the first of these now historic 
verifications. But we in our day may emphatically 
term the epoch of the third verified return Annus 
Halleianus. 

Says Turner, Halley's successor in the Savilian 
chair at Oxford to-day: "There can be no more 
complete or more sensational proof of a scientific 
law, than to predict events by means of it. Halley 
was deservedly the first to perform this great service 
for Newton's Law of Gravitation, and he would 
have rejoiced to think how conspicuous a part Eng- 
land was to play in the subsequent prediction of 
the existence of Neptune." 

Halley rose rapidly among the chief astronomical 
figures of his day. But he had little veneration for 
mere authority, and the significant veering of his 
religious views toward heterodoxy was for years 
an obstacle to his advance. 

Still Halley the astronomer was great enough to 
question any contemporary dicta that seemed to 
rest on authority alone. Everyone called the stars 
"fixed" stars; but Halley doubting this, made the 
first discovery of a star's individual motion — proper 
motion, as astronomers say. To-day, two hundred 
years after, every star is considered to be in motion, 
and astronomers are ascertaining their real motions 



HALLEY AND HIS COMET 87 

in the celestial spaces to a nicety undreamed of by 
even the exacting Halley. 

The moon, of priceless service to the early navi- 
gator, was regarded by all astronomers as endowed 
with an average rate of motion round the earth 
that did not vary from age to age. But Halley 
questioned this too; and on comparing v/ith the 
ancient value from Chaldean eclipses, he m.ade an- 
other discovery — the secular acceleration of the 
moon's mean motion, as it is technically termed. 
This was a colossal discovery in celestial dynamics ; 
and the reason underlying it lay hidden in Nev/ton's 
law for yet another century, till the keener mathe- 
matics of Laplace detected its true origin. 

With Newton, Halley laid down the firm foun- 
dations of celestial mechanics, and they pushed the 
science as far as the mathematics of their day 
would permit. Halley, however, v\^as not content 
with elucidating the motion of bodies nearest the 
earth, and pressed to the utmost confines of the 
solar system known to him. Here, too, he made a 
signal discovery of that mutual disturbance of the 
planets in their motion round the sun, called the 
great inequality of Jupiter and Saturn. 

Halley's versatile genius attacked all the great 
problems of the day. His observation of the sun's 
total eclipse in 1715 is the earliest reliable account 
of such a phenomenon by a trained astronomer. 
He described the corona minutely and was the first 
to see that other interesting phenomenon which 
only an alert observer can detect, which a great 
astronomer of a later day compared to the "ignition 
of a fine train of gunpowder," and which has ever 
since borne the name of "Bailey's beads." 



88 ASTRONOMY TO-DAY 

Besides being a great astronomer, Halley was a 
man of affairs as well, which Newton, although the 
greater mathematician, was not. Without Halley, 
Newton's superb discovery might easily have been 
lost to the age and nation, for the latter was bent 
merely on making discoveries, and on speculative 
contemplation of them, with never a thought of 
publishing to the world. 

Halley, more practical and businesslike, insisted 
on careful writing out and publication. Newton 
was then only forty-two, and Halley fully fourteen 
years his junior. But the philosophers of that day 
were keenly aHve to the mystery of Kepler's laws, 
and Halley was fully conscious of the grandeur and 
far-reaching significance of Newton's great gen- 
eralization which embodied all three of Kepler's laws 
in one. 

Newton at last yielded, though reluctantly, and 
the 'Trincipia" was given to the world, though 
wholly at Halley's private charges. 

But Halley was far from being com^pletely en- 
grossed with the absorbing problems of the sky; 
things terrestrial held for years his undivided at- 
tention. Imagine present-day Lords Commissioners 
of the Admiralty intrusting a ship of the British 
navy to civilian command. Yet such was their con- 
fidence in Halley that he was commissioned as cap- 
tain of H. M.'s pink Paramour in 1698, with instruc- 
tions to proceed to southern seas for geographical 
discoveries, and for improving knowledge of the 
longitude problem, and of the variations of the com- 
pass. Trade winds and monsoons, charts of mag- 
netic variation, tides and surveys of the Channel 
coast, and experiments with diving bells were prac- 
tical activities that occupied his attention. 



HALLEY AND HIS COMET 89 

Halley in 1720 became Astronomer Roj^al. He 
was the second incumbent of this great office, but 
the first to supply the Royal Observatory with in- 
struments of its own, some of which adorn its walls 
even to-day. His long series of lunar observations 
and his magnetic researches were of immense 
practical value in navigation. 

Halley lived to a ripe old age and left the world 
vastly better than he found it. His rise from hum- 
blest obscurity was most remarkable, and he lived 
to gratify all the ambitions of his early manhood. 
"Of attractive appearance^ pleasing manners, and 
ready wit," says one of his biographers, "loyal, 
generous, and free from self-seeking, he was one 
of the most personally engaging men who ever held 
the office of Astronomer Royal. 

He died in office at Greenwich in 1742. 

"Halley was buried," says Chambers, "in the 
churchyard of St. Margaret's, Lee, not far from 
Greenwich, and it has lately been announced that 
the Admiralty have decided to repair his tomb at 
the public expense, no descendants of his being 
known." There is no suitable monument in England 
to the memory of one of her greatest scientific men. 
In any event the collection and republication of 
his epoch-making papers would be welcomed by 
astronomers of every nation. 



CHAPTER XVII 

BRADLEY AND ABERRATION 

LIVING at Kew in London early in the 18th cen- 
^ tury was an enthusiastic young astronomer, 
James Bradley. He is famous chiefly for his accurate 
observations of star places which have been invalu- 
able to astronomers of later epochs in ascertaining 
the proper motions of stars. 

The latitude of Bradley's house in Kew was very 
nearly the same as the declination of the bright star 
Gamma Draconis, so that it passed through his 
zenith once every day. Bradley had a zenith sector, 
and with this he observed with the greatest care the 
zenith distance of Gamma Draconis at every possible 
opportunity. This he did by pointing the telescope 
on the star and then recording the small angle of its 
inclination to a fine plumb line. So accurate were 
his measures that he was probably certain of the 
star's position to the nearest second of arc. 

What he hoped to find was the star's motion round 
a very slight orbit once each year, and due to the 
earth's motion in its orbit round the sun. In other 
words, he sought to find the star's parallax if it 
turned out to be a measurable quantity. 

It is just as well now that his method of observa- 
tion proved insufficiently delicate to reveal the paral- 
lax of Gamma Draconis ; but his assiduity in observa- 
tion led him to an unexpected discovery of greater 
moment at that time. What he really found was 

90 



BRADLEY AND ABERRATION 91 

that the star had a regular annual orbit ; but wholly 
different from what he expected, and very much 
larger in amount. This result was most puzzling to 
Bradley. The law of relative motion would require 
that the star's motion in its expected orbit should 
be opposite to that of the earth in its annual orbit; 
instead of which the star was all the time at right 
angles to the earth's motion. 

Bradley was a frequent traveler by boat on the 
Thames, and the apparent change in the direction 
of the wind when the boat was in motion is said to 
have suggested to him what caused the displacement 
of Gamma Draconis. The progressive motion of 
light had been roughly ascertained by Roemer: let 
that be the velocity of the wind. And the earth's 
motion in its orbit round the sun, let that be the 
speed of the boat. Then as the wind (to an observer 
on the moving boat) always seems to come from a 
point in advance of the point it actually proceeds 
from (to an observer at rest) , so the star should be 
constantly thrown forward by an angle given by the 
relation of the velocity of light to the speed of the 
earth in orbital revolution round the sun. 

The apparent places of all stars are affected in 
this manner, and this displacement is called the 
aberration of light. Astronomers since Bradley's 
discovery of aberration in 1726 have devoted a great 
deal of attention to this astronomical constant, as it 
is called, and the arc value of it is very nearly 20".5. 
This means that light travels more than ten thousand 
times as fast as the earth in its orbit (186,330 miles 
per second as against the earth's 18.5) . And we can 
ascertain the sun's distance by aberration also be- 
cause the exact values of the velocity of light and of 
the constant of aberration v/hen properly combined 



92 ASTRONOMY TO-DAY 

give the exact orbital speed of the earth; and this 
furnishes directly by geometry the radius of the 
earth's orbit, that is the distance of the sun. 

In fact, this is one of the more accurate modern 
methods of ascertaining the distance of the sun. As 
early as 1880 it enabled the writer to calculate the 
sun's parallax equal to 8'''.80, a value absolutely 
identical with that adopted by the Paris Confer- 
ence of 1896, and now universally accepted as the 
standard. 

In whatever part of the sky we observe, every star 
is affected by aberration. At the poles of the ecliptic, 
23% degrees from the earth's poles, the annual aber- 
ration orbits of the stars are very small circles, 41" 
in diameter. Toward the ecliptic the aberration 
orbits become more and more oval, ellipses in fact 
of greater and greater eccentricity, but with their 
major axes all of the same length, until we reach the 
ecliptic itself; and then the ellipse is flattened into 
a straight line 41" in length, in which the star travels 
forth and back once a year. Exact correspondence 
of the aberration ellipses of the stars with the annual 
motion of the earth round the sun affords indis- 
putable proof of this motion, and as every star par^ 
takes of the movement, this proof of our motion 
round the sun becomes many million fold. 

Indeed, if we were to push a little farther the re- 
finement of our analysis of the effect of aberration 
on stellar positions, we could prove also the rotation 
of the earth on its axis, because that motion is swift 
enough to bear an appreciable ratio to the velocity 
of light. Diurnal aberration is the term applied to 
this slight effect, and as every star partakes of it, 
demonstration of the earth's turning round on its 
axis becomes many millionfold also. 



CHAPTER XVm 

THE TELESCOPE 

HAD anyone told Ptolemy that his earth-centered 
system of sun, moon, and stars would ultimately 
be overthrown, not by philosophy but by the over- 
whelming evidence furnished by a little optical in- 
strument which so aided the human eye that it could 
actually see systems of bodies in revolution round 
each other in the sky, he would no doubt have 
vehemently denied that any such thing was possible. 
To be sure, it took fourteen centuries to bring this 
about, and the discovery even then was v/ithout 
much doubt due to accident. 

Through all this long period when astronomy may 
be said to have merely existed, practically without 
any forward step or development, its devotees were 
unequipped with the sort of instruments which were 
requisite to make the advance possible. There were 
astrolabes and armillary spheres, with crudely di- 
vided circles, and the excellent work done with them 
only shows the genius of many of the early astrono- 
mers who had nothing better to work with. Re- 
garding star-places made with instruments fixed in 
the meridian, Bessel, often called the father of prac- 
tical astronomy, used to say that, even if you pro- 
vided a bad observer with the best of instruments, 
a genius could surpass him with a gun barrel and a 
cart wheel. 

Before the days of telescopes, that is, prior to the 
seventeenth century, it was not known v/hether any 

93 



94 ASTRONOMY TO-DAY 

of the planets except the earth had a moon or not ; 
consequently the masses of these planets were but 
very imperfectly ascertained ; the phases of Mercury 
and Venus were merely conjectured; what were the 
actual dimensions of the planets could only be 
guessed at ; the approximate distances of sun, moon, 
and planets were little better than guesses ; the dis- 
tances of the stars were wildly inaccurate; and the 
positions of the stars on the celestial sphere, and of 
sun, moon, and planets among them were far re- 
moved from modern standards of precision — all be- 
cause the telescope had not yet become available as 
an optical adjunct to increase the power of the 
human eye and enable it to see as if distances were 
in considerable measure annihilated. 

Galileo almost universally is said to have been the 
inventor of the telescope, but intimate research into 
the question would appear to give the honor of that 
original invention to another, in another country. 
What Galileo deserves the highest praise for, how- 
ever, is the reinvention independently of an "op- 
tick tube" by which he could bring distant objects 
apparently much nearer to him; and being an as- 
tronomer, he was by universal acknowledgment first 
of all men to turn a telescope on the heavenly bodies. 
This was in the year 1609, and his first discovery 
was the phase of Venus, his second the four Medicean 
moons or satellites of Jupiter, discoveries which at 
that epoch were of the highest significance in es- 
tablishing the truth of the Copernican system be- 
yond the shadow of doubt. 

But the first telescopes of which we have record 
were made, so far as can now be ascertained, in 
Holland very early in the 17th century. Metius, a 
professor of mathematics, and Jansen and Lipper- 



THE TELESCOPE 95 

hey, Yiho v/ere opticians in Middeiburg — all three 
are entitled to consideration as claimants of the orig- 
inal invention of the telescope. But that such an 
instrument was pretty well known would appear to 
be shown by his government's refusal of a patent to 
Lipperhey in 1608 ; while the officials recognizing the 
value of such an instrument for purposes of war, 
got him to construct several telescopes and ordered 
him to keep the invention a secret. 

Within a year Galileo heard that an instrument 
was in use in Holland by which it was possible to 
see distant objects as if near at hand. Skilled in 
optics as he v/as, the reinvention was a task neither 
long nor difficult for Mm. One of his first instru- 
ments magnified but three times ; still it made a great 
sensation in Venice where he exhibited the little 
tube to the authorities of that city, in which he first 
invented it. 

Galileo's telescope was of the simplest type, with 
but two lenses; the one a double convex lens with 
which an image of the distant object is formed, the 
other a double concave lens, much smaller which 
was the eye-lens for examining the image. It is this 
simple form of Galilean telescope that is still used 
in opera glasses and field glasses, because of the 
shorter tube necessary. 

Galileo carried on the construction of telescopes, 
all the time improving their quality and enlarging 
their power until he built one that magnified thirty 
times. What the diameter of the object glass was 
we do not know, perhaps two inches or possibly a 
little more. Glass of a quality good enough to make 
a telescope of cannot have been abundant or even 
obtainable except with great difficulty in those early 
days. 



96 ASTRONOMY TO-DAY 

Other discoveries by this first of celestial ob- 
servers were the spots on the sun, the larger moun- 
tains of the moon, the separate stars of which the 
Milky Way is composed, and, greatest wonder of all, 
the anomalous "handles" (ansae, he called them) of 
Saturn, which we now know as the planet's ring, the 
most wonderful of all the bodies in the sky. 

Since Galileo's time, only three centuries past, 
the progress in size and improvement in quality 
of the telescope have been marvelous. And 
this advance would not have been possible except 
for, first, the discoveries still kept in large part 
secret by the makers of optical glass which have 
enabled them to make disks of the largest size; 
second, the consummate skill of modern opticians 
in fashioning these disks into perfect lenses; and 
third, the progress in the mechanical arts and en- 
gineering, by which telescope tubes of many tons' 
weight are mounted or poised so delicately that the 
thrust of a finger readily swerves them from one 
point of the heavens to another. 

As the telescope is the most important of all 
astronomical instruments, it is necessary to 
understand its construction and adjustment and 
how the astronomer uses it. Telescopes are optical 
instruments, and nothing but optical parts would be 
requisite in making them, if only the optical con- 
ditions of their perfect working could be obtained 
without other mechanical accessories. 

In original principle, all telescopes are as simple 
as Gahleo's; first, an object glass to form the image 
of the distant object; second the eyepiece usually 
made of two lenses, but really a microscope, to 
magnify that image, and working in the same way 
that any microscope magnifies an object close at 






The 150-ft. Tower at the Mt. Wilson Solar Observatory. At the 

left is a diagram of tower, telescope and pit. ' At the upper right is an 

exterior view of the tower ; below a view looking down into the pit, 75 

ft. deep. (Photo, Mt. Wilson Solar Observatory.) 



THE TELESCOPE 97 

hand; and third, a tube to hold all the necessary- 
lenses in the true relative positions. 

The focal lengths of object glass and eyepiece 
will determine just what distance apart the lenses 
must be in order to give perfect vision. But it is 
quite as important that the axes of all the lenses be 
adjusted into one and the same straight line, and 
then held there rigidly and permanently. Otherwise 
vision with the telescope will be very imperfect 
and wholly unsatisfactory. The distance from the 
objective, or object glass to its focal point is called 
its focal length; and if we divide this by the focal 
length of the eyepiece, we shall have the magnifying 
power of the telescope. The eyepiece will usually 
be made of two lenses, or more, and we use its focal 
length considered as a single lens, in getting the 
magnifying power. A telescope will generally have 
many eyepieces of different focal lengths, so that 
it will have a corresponding range of magnifying 
powers. The lowest magnifying power will be not 
less than four or five diameters for each inch of 
aperture of the objective ; otherwise the eye will fail 
to receive all the light which falls upon the glass. 
A 4-inch telescope will therefore have no eyepiece 
with a lower magnifying power than about 20 
diameters. The highest magnifying power advan- 
tageous for a glass of this size will be about 250 to 
800, the working rule being about 70 diameters to 
each inch of aperture, although the theoretical 
limit is regarded as 100. 

The reason for a variety of eyepieces with dif- 
ferent magnifying powers soon becomes apparent 
on using the telescope. Comets and nebulae call for 
very low powers, while double stars and the plane- 
tary surfaces require the higher powers, provided 

Sci. Vol. 2-— 4 



98 ASTRONOMY TO-DAY 

the state of the atmosphere at the moment will 
allow it. If there is much quivering and unsteadi- 
ness, nothing is gained by trying the higher powers, 
because all the waves of unsteadiness are magnified 
also in the same proportion, and sharpness of vision, 
or fine definition, or "good seeing," as it is called, 
becomes impossible. The vibrations and tremors of 
the atmosphere are the greatest of all obstacles to 
\ astronomical observation, and the search is always 
' in order for regions of the world, in deserts or on 
high mountains, where the quietest atmosphere is 
to be found. 

Quite another power of the telescope is dependent 
on its objective solely: its light-gathering power. 
Light by which we see a star or planet is admitted 
to the retina of the eye through an adjustable aper- 
ture called the pupil. In the dark or at night, the 
pupil expands to an average diameter of one-fourth 
of an inch. But the object-glass of a telescope, by 
focusing the rays from a star, pours into the eye, 
almost as a funnel acts with water, all the light 
which falls on its larger surface. And as geometry 
has settled it for us that areas of surfaces are pro- 
portioned to the squares of their diameters, a two- 
inch object glass focuses upon the retina of the eye 
64 times as much light as the unassisted eye would 
receive. And the great 40-inch objective of the 
Yerkes telescope would, theoretically, yield 25,600 
times as much light as the eye alone. But there 
would be a noticeable percentage of this lost through 
absorption by the glasses of the telescope and 
scattering by their surfaces. 

The first makers of telescopes soon encountered 
a most discouraging difficulty, because it seemed to 
them absolutely insuperable. This is known as 



THE TELESCOPE 99 

chromatic aberration, or the scattering of light in 
a telescope due simply to its color or wave length. 
When light passes through a prism, red is refracted 
the least and violet the most. Through a lens it is 
the same, because a lens may be regarded as an in- 
definite system of prisms. The image of a star or 
planet, then, formed by a single lens cannot be 
optically perfect ; instead it will be a confused inter- 
mingling of images of various colors. With low 
powers this will not be very troublesome, but great 
indistinctness results from the use of high magnify- 
ing powers. 

The early makers and users of telescopes in the 
latter part of the seventeenth century found that the 
troublesome effects of chromatic aberration could 
be much reduced by increasing the focal length of 
the objective. This led to what we term engineer- 
ing difficulties of a very serious nature, because the 
tubes of great length were very awkward in point- 
ing toward celestial objects, especially near the 
zenith, where the air is quietest. And it was next 
to impossible to hold an object steadily in the field, 
even after all the troubles of getting it there had 
been successfully overcome. 

Bianchini and Cassini, Hevelius and Huygens 
were among the active observers of that epoch who 
built telescopes of extraordinary length, a hundred 
feet and upward. One tube is said to have been 
built 600 feet in length, but quite certainly it could 
never have been used. So-called aerial telescopes 
were also constructed, in which the objective was 
mounted on top of a tower or a pole, and the eyepiece 
moved along near the ground. But it is difficult to 
see how anything but fleeting glimpses of the 
heavenly bodies could have been obtained with such 



100 ASTRONOMY TO-DAY 

contrivances, even if the lenses had been perfect. 
Newton indeed, who was expert in optics, gave up 
the problem of improving the refracting telescope^ 
and turned his energies toward the reflector. 

In 1733, half a century after Newton and a cen- 
tury and a quarter after Galileo, Chester More Hall, 
an Englishman, found by experiment that chro- 
matic aberration could be nearly eliminated by 
making the objective of two lenses instead of one, 
and the same invention was made independently by 
Dollond, an English optician, who took out letters 
patent about 1760. So the size of telescopes seemed 
to be limited only by the skill of the glassmaker 
and the size of disks that he might find it practi- 
cable to produce. 

What Hall and Dollond did was to make the outer 
or crown lens of the objective as before, and place 
behind it a plano-concave lens of dense flint glass. 
This had the effect of neutralizing the chromatic 
effect, or color aberration, while at the same time 
only part of the refractive effect of the crown lens 
was destroyed. This ingenious but costly combina- 
tion prepared the way for the great refracting 
telescopes of the present day, because it solved, or 
seemed to solve, the important problem of getting 
the necessary refraction of light rays without harm- 
ful dispersion or decomposition of them. 

Through the 18th century and the first years of 
the 19th many telescopes of a size very great for 
that day were built, and their success seemed com- 
plete. With large increase in the size of the disks, 
however, a new trouble arose, quite inherent in the 
glass itself. The two kinds of glass, flint and 
crown, do not decompose white light with uniform- 
ity, so that when the so-called achromatic objec- 



THE TELESCOPE 101 

tive was composed of flint and crown, there was an 
effect known as irrationality of dispersion, or 
secondary spectrum, which produced a very trouble- 
some residuum of blue light surrounding the images 
of bright objects. This is the most serious defect 
of all the great refractors of the day, and effectively 
it limits their size to about 60 inches of aperture, 
with present types of flint and crown. It is ex- 
pected by present experimenters, however, that 
further improvements in optical glass will do much 
to extend this limit; so that a refracting telescope 
of much greater size than any now in existence will 
be practicable. 

Improvements in mounting telescopes, too, are 
still possible. Within recent years, Hartness, of 
Springfield, Vermont, has erected a new and ingen- 
ious type of turret telescope which protects the ob- 
server from wind and cold while his instrument is 
outside. It affords exceptional facilities for rapid 
and convenient observing, as for variable stars, and 
is adaptable to both refractors and reflectors. 

The captivating study of the heavens can of 
course be begun with the naked eye alone, but very 
moderate optical assistance is a great lielp and 
stimulates. An opera-glass affords such assistance; 
a field-glass does still better, and best of all, for 
certain purposes, is a modern prism-binocular. 



CHAPTER XIX 

REFLECTORS— MIRROR TELESCOPES 

CHERISHED with the utmost care in the rooms 
of the Royal Society of London is a world- 
famous telescope, a diminutive reflector made by the 
hands of Sir Isaac Newton. We have already men- 
tioned his connection with the refractor; and how 
he abandoned that type of telescope in favor of the 
reflecting mirror, or reflector in which the obstacles 
to great size appeared to be purely mechanical. By 
many, indeed, Newton is regarded as the inventor 
of the reflector. 

By the principles of optics, all the rays from a 
star that strike a concave mirror will be reflected 
to the geometric focal point, provided a section of 
that mirror is a parabola. Such a mirror is called 
a speculum, and is an alloy of tin, copper, and bis- 
muth. Its surface takes a Very high polish, reflect- 
ing when newly polished nearly 90 per cent of the 
light that falls upon it. 

But the focus where the eyepiece must be used is 
in front of the mirror, and if the eye were placed 
there, the observer's head would intercept all or 
much of the light that would otherwise reach the 
mirror. Gregory, probably the real inventor of the 
reflector, was the first to dodge this difficulty by 
perforating the mirror at the center and applying 
the eyepiece there, at the back of the speculum; 
but it was necessary to first send the rays to that 

102 



MIRROR TELESCOPES 103 

point by reflection from a second or smaller mirror, 
in the optical axis of the speculum. This reflects 
the rays backward down the tube to the eyepiece, or 
spectroscope, or camerao 

Another English optician, Cassegrain, improved 
on this design somewhat by placing the secondary 
mirror inside the focus of the speculum, or nearer 
to it, so that the tube is shorter. This form is pref- 
erable for many kinds of astronomical work, es- 
pecially photography. Herschel sought to do away 
with the secondary reflector entirely and save the 
loss of light by tilting the speculum slightly, so as 
to throw the image at one side of the tube ; but this 
modification introduces bad definition of the image 
and has never been much used. 

A better plan is that of Newton, who placed a 
small plane speculum at an angle of 45 degrees in 
the optical axis where the secondary mirror of the 
Gregory-Cassegrainian type is placed. The rays are 
then received by the eyepiece at the side of the upper 
end of the tube, the observer looking in at right 
angles to the axis. And a modern improvement first 
used by Draper is a small rectangular prism in place 
of the little plane speculum, effecting a saving of 
five to ten per cent of the light. 

It is not easy to say which type of telescope, the 
refractor or the reflector, is the more famous. Nor 
which is the better or more useful, or the more 
likely to lead in the astronomy of the future. When 
the successors of Dollond had carried the achro- 
matic refractor to the limit enforced by the size of 
the glass disks they were able to secure, they found 
these instruments not so great an improvement 
after all. The single-lens telescopes of great focal 
length were nearly as good optically, though much 



104 ASTRONOMY TO-DAY 

more awkward to handle. But the quality of the 
glass obtainable in that day appeared to set an 
arbitrary limit to that great amplification of size 
and power which progress in observational as- 
tronomy demanded. 

Then came the elder Herschel, best known and 
perhaps the greatest of all astronomers. At Bath, 
England, music was his profession, especially the 
organ. But he was dissatisfied with his little 
Gregorian reflector, and being a very clever me- 
chanician he set out to build a reflector for himself. 
It is said that he cast and polished nearly 200 
mirrors, in the course of experiments on the most 
highly reflective type of alloys, and the sort of 
mechanism that would enable him to give them the 
highest polish. In all his work he was ably and 
enthusiastically aided by his sister, Caroline Her- 
schel, most famous of all women astronomers. 

Upward in size of his mirrors he advanced, till 
he had a speculum of two feet diameter with a tube 
20 feet long. Twelve to fifteen years had elapsed 
when in 1781, while testing one of these reflectors 
on stars in the constellation Gemini, he made 
the first discovery of a planet since the invention 
of the telescope-— the great planet now known as 
Uranus. 

Under the patronage of King George, he advanced 
to telescopes of still greater size, his largest being 
no less than forty feet in length, with a speculum of 
four feet in diameter. Two new satellites of 
Saturn were discovered with this giant reflector, 
which was dismantled by Sir John Herschel with 
appropriate ceremonies, including the singing of an 
ode by the Herschel family assembled inside of the 
tube, on New Year's Eve, 1839-40. 



MIRROR TELESCOPES 105 

We have record of but few attempts to improve 
the size and definition of great reflectors by the 
continental astronomers during this era. In Eng- 
land and Ireland, however, great progress was 
made. About 1860 Lassell built a two-foot reflector, 
with which he discovered two new satellites of 
Uranus, and which he subsequently set up in the 
island of Malta. Ten years later Thomas Grubb 
and Son of Dublin constructed a four-foot reflector, 
now at the Observatory in Melbourne, Australia. 
Calver in conjunction with Common of Esling, Lon- 
don, about 1880-95 built several large reflectors, the 
largest of five feet diameter, now owned by Harvard 
College Observatory; and, rather earlier, Martin of 
Paris completed a four-foot reflector. 

The mirrors of these latter instruments were not 
made of speculum metal, but of solid glass, which 
must be very thick (one-seventh their diameter) in 
order to prevent flexure or bending by their own 
weight. So sensitive is the optical surface to dis- 
tortion that unless a complicated series of levers 
and counterpoises is supplied, to support the under 
surface of the mirror, the perfection of its optical 
figure disappears when the telescope is directed to 
objects at different altitudes in the sky. The upper 
or outer surface of the glass is the one which re- 
ceives the optical polish on a heavy coat of silver 
chemically deposited on the polished glass after its 
figure has been tested and found satisfactory. 

But far and away the most famous reflecting 
telescope of all is the "Leviathan'' of Lord Rosse, 
built at Birr Castle, Parsonstown, Ireland, about the 
middle of the last century. His Lordship made 
many ingenious improvements in grinding the 
mirror, which was of speculum metal, six feet in 



106 ASTRONOMY TO-DAY 

diameter and weighed seven tons. It was ground 
to a focal length of fifty-four feet and mounted be- 
tween heavy walls of masonry, so that the motion of 
the great tube was restricted to a few degrees on 
both sides of the meridian. The huge mechanism 
was very cumbersome in operation, and photog- 
raphy was not available in those days ; nevertheless 
Lord Rosse's telescope made the epochal discovery 
of the spiral nebulae, which no other telescope of 
that day could have done. 

In America the reflector has always kept at least 
even pace with the refractor. As early as 1830, 
Mason and Smith, two students at Yale College, 
enthused by Denison Olmsted, built a 12-inch specu- 
lum with which they made unsurpassed observations 
of the nebula. Dr. Henry Draper, returning from a 
visit to Lord Rosse, began about 1865 the construc- 
tion of two silver-on-glass reflectors, one of 15 
inches diameter, the other of 28 inches, with which 
he did important work for many years in photog- 
raphy and spectroscopy, and his mirrors are now 
the property of Harvard College Observatory. Alvan 
Clark and Sons have in later years built a 40-inch 
mirror for the Lowell Observatory in Arizona, and 
very recently a 6-foot silver-on-glass mirror has 
been set up in the Dominion of Canada Astrophy- 
sical Observatory at Victoria, British Columbia, 
where it is doing excellent work in the hands of 
Plaskett, its designer. 

The huge glass disk for the reflector weighs two 
tons, and it must be cast so that there are no in- 
ternal strains; otherwise it is liable to burst in 
fragments in the process of grinding. It should be 
free from air-bubbles, too; so the glass is cast in 
one melting, if possible. This disk was made by 



MIRROR TELESCOPES 107 

the St. Gobain Plate Glass Company, whose works 
have been ruthlessly destroyed by the enemy during 
the war; but fortunately the great disk had been 
shipped from Antwerp only a week before declara- 
tion of hostilities. 

Brashear of Allegheny was intrusted with the 
optical parts, which occupied many months of 
critical work. The finished mirror is 73 inches in 
diameter, its focal length is 30 feet, and its thickness 
12 inches. A central hole 10 inches in diameter 
makes possible its use as a Gregorian or Cassegrain- 
ian type, as well as Newtonian. The mechanical parts 
of this great telescope are by Warner and Swasey of 
Cleveland, after the well-known equatorial mount- 
ing of the Melbourne reflector by Grubb of Dublin. 
Friction of the polar and declination axes is re- 
duced by ball bearings. The 66-foot dome has an 
opening 15 feet wide and extending six feet beyond 
the zenith. All motions of the telescope, dome 
shutters, and observing platform are under com- 
'plete control by electric motors. Spectroscopic 
binaries form one of the special fields of research 
with this powerful instrument, and many new bin- 
aries have already been detected. 

The great reflectors designed and constructed by 
Ritchey, formerly of Chicago and now of Pasadena, 
deserve especial mention. While connected with 
the Yerkes Observatory he constructed a two-foot 
reflector for that institution, with which he had ex- 
ceptional success in photography of the stars and 
nebulae. Later he built a 5-foot reflector, now at 
the Carnegie Observatory on Mount Wilson, Cali- 
fornia, with which the spiral nebulae and many 
other celestial objects have been especially well 
photographed. Ritchey's later years have been 



108 ASTRONOMY TO-DAY 

spent on the construction of an even greater mirror, 
no less than 100 inches in diameter, which was com- 
pleted in 1919, and has already yielded photographic 
results dealt with farther on, and far surpassing 
anything previously obtained. Theoretically this 
huge mirror, if its surface were perfectly reflective 
so that it would transmit all the rays falling upon 
it, would gather 16,000 times as much light as the 
unaided eye alone. 

Whether a 72-inch refractor, should it ever be 
constructed, would surpass the 100-inch reflector as 
an all-round engine for astronomical research, is a 
question that can only be fully answered by build- 
ing it and trying the two instruments alongside. 

Probably three-quarters of all the really great 
astronomical work in the past has been done by re- 
fractors. They are always ready and convenient 
for use, and the optical surfaces rarely require 
cleaning and readjustment. With increase of size, 
however, the secondary spectrum becomes very 
bothersome in the great lenses ; and the larger they 
are, the more light is lost by absorption on account 
of the increasing thickness of the lenses. With the 
reflector on the other hand, while there is clearly 
a greater range of size, the reflective surface re- 
tains its high polish only a brief period, so that 
mere tarnish effectively reduces the aperture; and 
the great mirror is more or less ineffective in con- 
sequence of flexure uncompensated by the lever 
system that supports the back of the mirror. 

Both types of telescope still have their enthusi- 
astic devotees; and the next great reflector would 
doubtless be a gratifying success, if mounted in some 
elevated region of the world, like the Andes of 
northern Chile, where the air is exceptionally steady 



MIRROR TELESCOPES 109 

and the sky very clear a large part of the year. The 
highest magnifying powers suitable for work with 
such a telescope could then be employed, and new 
discoveries added as well as important work done in 
extension of lines already begun on the universe of 
stars. 

On the authority of Clark, even a six-foot objec- 
tive would not necessitate a combined thickness of 
its glasses in excess of six inches. Present disks 
are vastly superior to the early ones in transpar- 
ency, and there is reason to expect still greater im- 
provement. The engineering troubles incident to 
execution of the mechanical side of the scheme need 
not stand in the way; they never have, indeed the 
astronomer has but just begun to invoke the fertile 
resources of the modern engineer. Not long before 
his death the younger Clark who had just finished 
the great lenses of the 40-inch Yerkes telescope, 
ventured this prevision, already in part come true: 
"The new astronomy, as well as the old, demands 
more power. Problems wait for their solution, and 
theories to be substantiated or disproved. The hori- 
zon of science has been greatly broadened within 
the last few years, but out upon the borderland I 
see the glimmer of new lights that await for their 
interpretation, and the great telescopes of the future 
must be their interpreters." 

Practically all the great telescopes of the world 
have in turn signalized the new accession of power 
by some significant astronomical discovery: to 
specify, one of Herschel's reflectors first revealed 
the planet Uranus; Lord Rosse's "Leviathan" the 
spiral nebulae ; the 15-inch Cambridge lens the crape, 
or dusky ring of Saturn; the IS^-inch Chicago 
refractor the companion of Sirius ; the Washington 



110 ASTRONOMY TO-DAY 

26-inch telescope the satellites of Mars ; the 30-inch 
Pulkowa glass the nebulosities of the Pleiades; 
and the 36-inch Lick telescope brought to light a 
fifth satellite of Jupiter. At the time these dis- 
coveries were made, each of these great telescopes 
was the only instrument then in existence with 
power enough to have made the discovery possible. 
So we may advance to still farther accessions of 
power with the expectation that greater discoveries 
will continue to gratify our confidence. 



CHAPTER XX 

THE STORY OF THE SPECTROSCOPE 

SIR ISAAC NEWTON ought really to have been 
the inventor of the spectroscope, because he 
began by analyzing light in the rough with prisms, 
was very expert in optics, and was certainly enough 
of a philosopher to have laid the foundations of the 
science. 

What Newton did was to admit sunlight into a 
darkened room through a small round aperture, 
then pass the rays through a glass prism and re- 
ceive the band of color on a screen. He noticed the 
succession of colors correctly — violet, indigo, blue, 
green, yellow, orange, red; also that they were not 
pure colors, but overlapping bands of color. Ap- 
parently neither he nor any other experimenter for 
more than a century went any further, when the 
next essential step was taken by Wollaston about 
1802 in England. He saw that by receiving the light 
through a narrov/ slit instead of a round hole, he 
got a purer spectrum, spectrum being the name 
given to the succession of colors into which the 
prism splits up or decomposes the original beam of 
white sunlight. This seemingly insignificant change, 
a narrow slit replacing the round hole, made Wol- 
laston and not Newton the discoverer of the dark 
lines crossing the spectrum at various irregular in- 
tervals, and these singularly neglected lines meant 
the basis of a new and most important science. 

Ill 



112 ASTRONOMY TO-DAY 

Even Wollaston, however, passed them by, and 
it was Fraunhofer who in 1814-1815 first made a 
chart of them. Consequently they are known as 
Fraunhofer lines, or dark absorption lines. Send- 
ing the beam of light through a succession of prisms 
gives greater dispersion and increases the power of 
the spectroscope. The greater the dispersion the 
greater the number of absorption lines ; and it is the 
number and intensity of these lines, with their ac- 
curate position throughout the range of the spectrum 
which becomes the basis of spectrum analysis. 

The half century that saw the invention of the 
steam engine, photography, the railroad and the 
telegraph elapsed without any farther developments 
than mere mapping of the fundamental lines, A, B, 
C, D, E, F, G, H of the solar spectrum. The moon, 
too, was examined and its spectrum found the same, 
as was to be expected from sunlight simply reflected. 

Sir John Herschel and other experimenters came 
near guessing the significance of the dark lines, but 
the problem of unraveling their mystery was finally 
solved by Bunsen and Kirchhoff who ascertained 
that an incandescent gas emits rays of exactly the 
same degree of ref rangibility which it absorbs when 
white light is passed through it. This great dis- 
covery was at once received as the secure basis of 
spectrum analysis, and Kirchhoff in 1858 put in 
compact and comprehensive form the three follow- 
ing principles underlying the theory of the science : 

(1) Solid and liquid bodies, also gases under high 
pressure, give when incandescent a continous 
spectrum, that is one with a mere succession of 
colors, and neither bright nor dark lines ; 

(2) Gases under low pressure give a discontinuous 
spectrum, crossed by bright lines whose number and 



STORY OF THE SPECTROSCOPE 113 

position in the spectrum differ according to the sub- 
stances vaporized; 

(3) When white light passes through a gas, this 
medium absorbs or quenches rays of identical wave- 
length with those composing its own bright-line 
spectrum. 

Clearly then it makes no difference where the light 
originates whether it comes from sun or star. Only 
it must be bright enough so that we can analyze it 
with the spectroscope. But our analysis of sun and 
star could not proceed until the chemist had 
vaporized in the laboratory all the elements, and 
charted their spectra with accuracy. When this had 
been done, every substance became at once recogniz- 
able by the number and position of its lines, with 
practical certainty. 

How then can we be sure of the chemical and 
physical composition of sun and stars ? Only by de- 
tailed and critical comparison of their spectra with 
the laboratory spectra of elements which chemical 
and physical research have supplied. As in the sun, 
so in the stars, each of which is encircled by a 
gaseous absorptive layer or atmosphere, the light 
rays from the self-luminous inner sphere must pass 
through this reversing layer, which absorbs light of 
exactly the same wave length as the lines that make 
up its own bright line spectrum. Whatever sub- 
stances are here found in gaseous condition, the same 
will be evident by dark lines in the spectrum of sun 
or star, and the position of these dark lines will show, 
by coincidence with the position of the laboratory 
bright lines, all the substances that are vaporized in 
the atmospheres of the self-luminous bodies of the sky= 

Here then originated the science of the new as- 
tronomy: the old astronomy had concerned itself 



114 ASTRONOMY TO-DAY 

mainly with positions of the heavenly bodies, where 
they are ; the new astronomy deals with their chemi- 
cal composition and physical constitution, and what 
they are. Between 1865 and 1875 the fundamental 
application of the basic principles was well advanced 
by the researches of Sir William Huggins in Eng- 
land, of Father Angelo Secchi in Rome, of Jules 
Janssen in Paris, and of Dr. Henry Draper in New 
York. 

In analyzing the spectrum of the sun, many 
thousands of dark absorption lines are found, and 
their coincidences with the bright lines of terrestrial 
elements show that iron, for instance, is most prom- 
inently identified, with rather more than 2,000 co- 
incidences of bright and dark lines. Calcium, too, 
is indicated by peculiar intensity of its lines, as well 
as their great number. Next in order are hydrogen, 
nickel and sodium. By prolonged and minute com- 
parison of the solar spectrum with spectra of ter- 
restrial elements, something like forty elemental 
substances are now known to exist in the sun. Row- 
land's splendid photographs of the solar spectrum 
have contributed most effectively. About half of 
these elements, though not in order of certainty, are 
aluminum, cadmium, calcium, carbon, chromium, 
cobalt, copper, hydrogen, iron, magnesium, man- 
ganese, nickel, scandium, silicon, silver, sodium, 
titanium, vanadium, yttrium, zinc, and zirconium. 
Oxygen, too, is pretty surely indicated; but certain 
elements abundant on earth, as nitrogen and chlo- 
rine, together with gold, mercury, phosphorus, and 
sulphur, are not found in the sun. 

The two brilliant red stars, Aldebaran in Taurus, 
and Betelgeuse in Orion, were the first stars whose 
chemical constitution was revealed to the eye of man, 



STORY OF THE SPECTROSCOPE 115 

and Sir William Huggins of London was the astrono- 
mer who achieved this epoch-making result. Father 
Secchi of the Vatican Observatory proceeded at 
once with the visual examination of the spectra of 
hundreds of the brighter stars, and he was the first 
to provide a classification of stellar spectra. There 
were four types. 

Secchi's type I is characterized chiefly by the 
breadth and intensity of dark hydrogen lines, to- 
gether with a f aintness or entire absence of metallic 
lines. These are bluish or white stars and they are 
very abundant, nearly half of all the stars. Vega, 
Altair, and numerous other bright stars belong to 
this type, and especially Sirius, which gives to the 
type the name "Sirians." 

Type II is characterized by a multitude of fine 
dark metallic lines, closely resembling the lines of 
the solar spectrum. These stars are somewhat 
yellowish in tinge like the sun, and from this 
similarity of spectra they are called "solars." 
Arcturus and Capella are "solars," and on the whole 
the solars are, rather less numerous than the Sirians. 
Stars nearest to the solar system are mostly of this 
type, and, according to Kapteyn of Groningen, the 
absolute luminous power of first type stars exceeds 
that of second type stars seven fold. 

Secchi's type III is characterized by many dark 
bands, well defined on the side toward the blue end 
of the spectrum, but shading off toward the red— 
a "colonnaded spectrum", as Miss Gierke aptly terms 
it. Alpha Herculis, Antares, and Mira, together 
with orange and reddish stars and most of the 
variable stars, belong in type III. 

Type IV is also characterized by dark bands, often 
called "flutings," similar to those of type III, but 



116 ASTRONOMY TO-DAY 

reversed as to shading, that is, well defined on the 
side toward the red, but fading out toward the blue. 
Their atmospheres contain carbon ; but they are notr 
abundant, besides being faint and nearly all blood- 
red in tint. 

Following up the brilliant researches of Draper, 
who in 1872 obtained the first successful photograph 
of a star's spectrum, that of Vega, Pickering of 
Harvard supplemented Secchi's classification by 
Type V, a spectrum characterized by bright lines. 
They, too, are not abundant and are all found near 
the middle of the Galaxy. These are usually known 
as Wolf-Rayet stars, from the two Paris astrono- 
mers who first investigated their spectra. Type 
V stars are a class of objects seemingly apart from^ 
the rest of the stellar universe, and many of the 
planetary nebulae yield the same sort of a spectrum. 

The late Mrs. Anna Palmer Draper, widow of Dr. 
Henry Draper, established the Henry Draper Me- 
morial at Harvard, and investigation of the photo- 
graphic spectra of all the brighter stars of the 
entire heavens has been prosecuted on a compre- 
hensive scale, those of the northern hemisphere at 
Cambridge, and of the southern at Arequipa, Peru. 
These researches have led to a broad reclassifica- 
tion of the stars into eight distinct groups, a work 
of exceptional magnitude begun by the late Mrs. 
Fleming and recently completed by Miss Annie 
Cannon, who classified the photographic spectra of 
more than 230,000 stars on the new system, as fol- 
lows : — 

The letters 0, B, A, F, G, K, M, N represent a 
continuous gradation in the supposed order of stel- 
lar evolution, and farther subdivision is indicated by 
tenths, G5K meaning a type half way between G and 



STORY OF THE SPECTROSCOPE 117 

K, and usually written G5 simply. B2 would indi- 
cate a type between B and A, but nearer to B than 
A, and so on. On this system, the spectrum of a 
star in the earliest stages of its evolution is made 
up of diffuse bright bands on a faint continuous 
background. As these bands become fewer and 
narrower, very faint absorption lines begin to ap- 
pear, first the helium lines, followed by several series 
of hydrogen lines. On the disappearance of the 
bright bands, the spectrum becomes wholly ab- 
sorptive bands and lines. Then comes a very great 
increase in intensity of the true hydrogen spectrum, 
with wide and much diffused lines, and few if any 
other lines. Then the H and K calcium lines and 
other lines peculiar to the sun become more and 
more intense. Then the hydrogen lines go through 
their long decline. The calcium spectrum becomes 
intense, and later the spectrum becomes quite like 
that of the sun with a great wealth of lines. Fol- 
lowing this stage the spectrum shortens from the 
ultra violet, the hydrogen lines fade out still farther, 
and bands due to metallic compounds make their ap- 
pearance, the entire spectrum finally resembling that 
of sun spots. To designate these types rather more 
categorically : — 

Type — bright bands on a faint continuous back- 
ground, with five subdivisions, Oa, Ob, Oc, Od, Oe, 
according to the varying width and intensity of the 
bands. 

Type B — ^the Orion type, or helium type, with ad- 
ditional lines of origin unknown as yet, but without 
any of the bright bands of type 0. 

Type A — the Sirian type, the regular Balmer 
series of hydrogen lines being very intense, with 
a few other lines not conspicuously marked. 



118 ASTRONOMY TO-DAY 

Type F — the calcium type, hydrogen lines less 
strongly marked, but with the narrow calcium lines 
H and K very intense. 

Type G — the solar type, with multitudes of metal- 
lic lines. 

Type K— in some respects similar to G, but with 
the hydrogen lines fading out, and the metallic lines 
relatively more prominent. 

Type M — spectrum with peculiar flutings due to 
titanium oxide, with subdivisions Ma and Mb, and 
the variable stars of long period, with a few bright 
hydrogen lines additional, in a separate class Md. 

Type N — similar to M, in that both are pro- 
nouncedly reddish, but with characteristic flutings 
probably indicating carbon compounds. 

The Draper classification being based on photo- 
graphic spectra, and the original Secchi classifica- 
tion being visual, the relation of the two systems is 
approximately as follows: 

Secchi Type I includes Draper B & A 

II includes Draper F, G & K 

III includes Draper M 

IV includes Draper N 

Pickering's marked success in organization and 
execution of this great program.me was due to his 
adoption of the '*slitless spectroscope," which made 
it possible to photograph stellar spectra in vast 
numbers on a single plate. The first observers of 
stellar spectra placed the spectroscope beyond the 
focus of the telescope with which it was used, there- 
by limiting the examination to but one star at a 
time. In the slitless spectroscope, a large prism is 
mounted in front of the objective (of short focus), 
so that the star's rays pass through it first, and then 
are brought to the same focus on the photographic 



STORY OF THE SPECTROSCOPE 119 

plate, for all the stars within the field of view, some- 
times many thousand in number. This arrange- 
ment provides great advantages in the comparison 
and classification of stellar spectra. 

When spectroscopic methods were first intro- 
duced into astronomy, there was no expectation that 
the field of the old or so-called exact astronomy 
would be invaded. Physicists were sometimes 
jocularly greeted among astronomers as "ribbon 
men," and no one even dreamed that their researches 
were one day to advance to equal recognition with 
results derived from micrometer, meridian circle, 
and heliometer. 

The first step in this direction was taken in 1868 
by Sir William Huggins of London, who noticed 
small displacements in the lines of spectra of very 
bright stars. In fact the whole spectrum appeared 
to be shifted; in the case of Sirius it was shifted 
toward the red, while the whole spectrum of Arc- 
turus was shifted by three times this amount toward 
the violet end of the spectrum. The reason was not 
difficult to assign. 

As early as 1842 Doppler had enunciated the 
principle that when we are approaching or are ap- 
proached by a body which is emitting regular vibra- 
tions, then the number of waves we receive in a 
second is increased, and their wave-length corre- 
spondingly diminished; and just the reverse of this 
occurs when the distance of the vibrating body is 
increasing. It is the same with light as with sound, 
and everyone has noticed how the pitch of a loco- 
motive whistle suddenly rises as it passes, and falls 
as suddenly on retreating from us. So Huggins 
drew the immediate inference that the distance be- 
tween the earth and Sirius was increasing at the 



120 ASTRONOMY TO-DAY 

rate of nearly twenty miles per second, while Arc- 
turus was nearing us with a velocity of sixty miles 
per second. 

These pioneer observations of m^otions in the line 
of sight, or radial velocities as they are now called, 
led directly to the acceptance of the high value of 
spectroscopic work as an adjunct of exact astron- 
omy in stellar research. Nor has it been found 
wanting in application to a great variety of exact 
problems in the solar system which would have been 
wholly impossible to solve without it. 

Foremost is the sun, of course, because of the 
overplus of light. Young early measured the dis- 
placement of lines in the spectra of the prominences, 
and found velocities sometimes exceeding 250 miles 
per second. Many astronomers, Duner among them, 
investigated the rotation of the sun by the spectro- 
scopic method. The sun's east limb is coming to- 
ward us, while the west is going from us; and by 
measuring the sum of the displacements, the rate of 
rotation has been calculated, not only at the sun's 
equator but at many solar latitudes also, both north 
and south. As was to be expected, these results 
agree well with the sun's rotation as found by the 
transits of sun spots in the lower latitudes where 
they make their appearance. 

Belopolsky has applied the same method to the 
rotation of the planet Venus, and Keeler, by measur- 
ing the displacement of lines in the spectrum of 
Saturn, on opposite sides of the ring, provided a 
brilHant observational proof of the physical con- 
stitution of the rings ; because he showed that the 
inner ring traveled round more swiftly than the 
outer one, thus demonstrating that the ring could 
not be solid, but must be composed of multitudes of 



STORY OF THE SPECTROSCOPE 121 

small pai-ticles traveling around the ball of Saturn, 
much as if they were satellites. Indeed, Keeler as- 
certained the velocity of their orbital motion and 
found that in each case it agreed exactly v^ith that 
required by the Keplerian law. 

Even the filmy corona of the sun was investigated 
in similar fashion by Deslandres at the total eclipse 
of 1898, and he found that it rotates bodily with the 
sun. But the complete vindication of the spectro- 
scopic method as an adjunct of the old astronomy 
came with its application to measurement of the 
distance of the sun. The method is very interest- 
ing and was first suggested by Campbell in 1892. 
Spectrum-line measurements have become very ac- 
curate with the introduction of dry-plate photo- 
graphy, and ecliptic stars were spectrographed, to- 
ward and from which the earth is traveling by its 
orbital motion round the sun. By accurate measure- 
ment of these displacements, the orbital velocity of 
the earth is calculated; and as we know the exact 
length of the year, or a complete period, the length 
of the orbit itself in miles becomes known, and thus, 
by simple mensuration, the length of the radius of 
the orbit — which is the distance of the sun. 

If we pass from sun to star, the triumph of the 
spectroscope has been everywhere complete and sig- 
nificant. As the spectroscopic survey of the stars 
grew toward completeness, it became evident that 
the swarming hosts of the stellar universe are in 
constant motion through space, not only athwart 
the line of vision as their proper motions had long 
disclosed, but some stars are swiftly moving toward 
our solar system and others as swiftly from it. 

Fixed stars, strictly speaking — there are no such. 
All are in relative motion. Exact astronomy by dis- 



122 ASTRONOMY TO-DAY 

cussion of the proper motions had assigned a region 
of the sky toward which the sun and planets are 
moving. Spectrography soon verified this direc- 
tion not only, but gave a determination of the ve- 
locity of our motion of twelve miles per second in 
a direction approximately that of the constellation 
Lyra. From corresponding radial velocities, we 
draw the ready conclusion that certain groups or 
clusters of stars are actually connected in space and 
moving as related systems, as in the Pleiades and 
Ursa Major. 

Rather more than a quarter century ago, the 
spectroscope came to the assistance of the telescope 
in helping to solve the intricate problem of stellar 
distribution. Kapteyn, by combining the proper 
motions of certain stars with their classification in 
the Draper catalogue of stellar spectra, drew the 
conclusion that, as stars having very small proper 
motions show a condensation toward the Galaxy, the 
stars composing this girdle are mostly of the Sirian 
type, and are at vast distances from the solar sys- 
tem. The proper motion of a star near to us will 
ordinarily be large, and, in the case of solar stars, 
the larger their proper motion the greater their 
number. So it would appear that the solar stars 
are aggregated round the sun himself, and this con- 
clusion is greatly strengthened by the fact that of 
stars whose distances and spectral type are both 
ascertained, seven of the eight nearest to us are 
solar stars. 

In 1889 the spectroscope achieved an unexpected 
triumph by enabling the late Professor Pickering to 
make the first discovery of a spectroscopic double, or 
binary star, a type of object now quite abundant. 
Unlike the visual binary systems v/hose periods are 



STORY OF THE SPECTROSCOPE 123 

years in length, the spectroscopic binaries have short 
periods, reckoned in some cases in days, or hours 
even. If the orbit of a very close binary is seen 
edge on, the light of the two stars will coalesce twice 
in every revolution. Halfway between these points 
there are two times when the two stars will be mov- 
ing, one toward the earth and the other from it. At 
all times the light of the star, in so far as the tele- 
scope shows it, proceeds from a single object. 

Now photograph the star's spectrum at each of 
the four critical points above indicated : in the first 
pair the lines are sharply defined and single, because 
at conjunction the stars are simply moving athwart 
the line of sight, while at the intermediate points 
the lines are double. Doppler's principle completely 
accounts for this : the light from the receding com- 
panion is giving lines displaced toward the red, while 
the approaching companion yields lines displaced 
toward the violet. Mizar, the double star at the 
bend of the handle in the Great Dipper was the first 
star to yield this peculiar type of spectrum, and the 
period of its invisible companion is about 52 days. 
The relative velocity of the components is 100 miles 
a second, and applying Newton's law we find its 
mass exceeds that of the sun f ortyf old. Capella has 
been found to be a spectroscopic binary; also the 
pole star. Spectroscopic binaries have relatively 
short periods, one of the shortest known being only 
35 hours in length. It is in the constellation Scorpio. 
I Beta Aurigse is another whose lines double on 
alternate nights, giving a period of four days ; and 
the combined mass of both stars is more than twice 
that of the sun. The catalogue of spectroscopic 
binaries is constantly enlarging; but thousands 
doubtless exist that can never be discovered by this 



124 ASTRONOMY TO-DAY 

method, as is evident if their orbits are perpendicu- 
lar to the line of sight or nearly so. The history 
of the spectroscopic binaries is one of the most in- 
teresting chapters in astronomy, and affords a mar- 
velous confirmation of the prediction of Bessel who 
first wrote of "the astronomy of the invisible." 

Find a star's distance by the spectroscope? Im- 
possible, everyone would have said, even a very few 
years ago. Now, however, the thing is done, and 
with increasing accuracy. 

Adams of Mount Wilson has found, after pro- 
tracted investigation, that the relative intensity of 
certain spectral lines varies according to the absolute 
brightness of a star; indeed, so close is the cor- 
respondence that the spectroscopic observations are 
employed to provide in certain cases a good determi- 
nation of the absolute magnitude, and therefore of 
the distance. To test this relation, the spectroscopic 
parallaxes have been compared with the measured 
parallaxes in numerous instances, and an excellent 
agreement is shown. This new method is adding 
extensively to our knowledge of stellar luminosities 
and distances, and even the vast distances of globu- 
lar clusters and spiral nebulae are becoming known. 

In fact, but few departments of the old astronomy 
are left which the new astronomy has not invaded, 
and this latest triumph of the spectroscope in de- 
termining accurately the distances of even the re- 
motest stars is enthusiastically welcomed by ad- 
vocates of the old and new astronomy alike. 



CHAPTER XXI 

THE STORY OF ASTRONOMICAL 
PHOTOGRAPHY 

THE most powerful ally of both telescope and spec- 
troscope is photography. Without it the mar- 
velous researches carried on with both these types 
of instrument would have been essentially impos- 
sible. Even the great telescopes of Herschel and 
Lord Rosse, notwithstanding their splendid record 
as optical instruments, might have achieved vastly 
more had photography been developed in their time 
to the point where the astronomer could have em- 
ployed its wonderful capabilities as he does to-day. 
And, with the spectroscope, it is hardly too much to 
say that no investigator ever observes visually with 
that instrument any more: practically every spec- 
trum is made a matter of photographic record first. 
The observing, or nowadays the measuring, is all 
done afterward. 

All telescopes and cameras are alike, in that each 
must form or have formed within it an image by 
means of a lens or mirror. In the telescope the eye 
sees the fleeting image, in the camera the ^process of 
registering the image on a plate or film is known as 
photography. Daguerre first invented the process 
(silver film on a copper plate) in 1839. The year 
following it was first employed on the moon, in 1850 
the first star was photographed, in 1851 the first 
total eclipse of the sun ; all by the primitive daguer- 

125 



126 ASTRONOMY TO-DAY 

reotype process, which, notwithstanding its awk- 
wardness and the great length of exposure required, 
was found to possess many advantages for astro- 
nomical work. 

About the middle of the last century the wet plate 
process, so called because the sensitized collodion 
film must be kept moist during exposure, came into 
general use, and the astronomers of that period were 
not slow to avail themselves of the advantages of 
a more sensitive process, which in 1872, in the 
skillful hands of Henry Draper, produced the first 
spectrum of a star. In 1880 a nebula was first 
photographed, and in 1881 a comet. 

Before this time, however, the new dry-plate proc- 
ess had been developed to the point where astrono- 
mers began to avail of its greater convenience and 
increased sensitiveness, even in spite of the coarse- 
ness of grain of the film. Forty years of dry-plate 
service have brought a wealth of advantages scarcely 
dreamed of in the beginning, and nearly every de- 
partment of astronomical research has been en- 
hanced thereby, while many entirely new photo- 
graphic methods of investigation have been worked 
out. 

Continued improvement in photographic proc- 
esses has provided the possibility of pictures of 
fainter and fainter celestial objects, and all the 
larger telescopes have photographed stars and 
nebula of such exceeding faintness that the human 
eye, even if applied to the same instrument, would 
never be able to see them. This is because the eye, 
in ten or twelve seconds of keen watching, becomes 
fatigued and must be rested, whereas the action of 
very faint light rays is cumulative on the highly 
sensitive film ; so that a continuous exposure of many 



ASTRONOMICAL PHOTOGRAPHY 127 

hours* duration becomes readily visible to the eye 
on development. So a supersensitive dry plate will 
often record many thousand stars in a region where 
the naked eye can see but one. 

Perhaps the greatest amplification of photography 
has taken place at the Harvard Observatory under 
Pickering, where a library of many hundred thou- 
sand plates has accumulated ; and at Groningen, Hol- 
land, where Kapteyn has established an astronomi- 
cal laboratory without instruments except such as 
are necessary to measure photographic plates, when- 
ever and wherever taken. So it is possible to 
select the clearest of skies, all over the world, for 
exposure of the plates, and bring back the photo- 
graphs for expert discussion. 

Of course the sun was the celestial body first 
photographed, and its surpassing brilliance neces- 
sitates reduction of exposure to a minimum. In 
moments of exceptional steadiness of the atmos- 
phere, a very high degree of magnification of the 
solar surface on the photographic plate is permitted, 
and the details in formation, development, and end- 
ing of sun spots are faithfully registered. Neverthe- 
less, it cannot be said that photography has yet 
entirely replaced the eye in this work, and careful 
.drawings of sun spots at critical stages in their life 
ire capable of registering fine detail which the plate 
las so far been unable to record. Janssen of Paris 
took photographs of the solar photosphere so highly 
lagnified that the granulation or willow-leaf struc- 
ture of the surface was clearly visible, and its varia- 
tions traceable from hour to hour. 

The advantages of sun spot photography in as- 
Icertaining the sun's rotation, keeping count of the 
spots, and in a permanent record for measurement 



128 ASTRONOMY TO-DAY 

of position of the sun's axis and the spot zones, are 
obvious. In direct portrayal of the sun's corona 
during total eclipses, photography has offered su- 
perior advantages over visual sketching, in the form 
and exact location of the coronal streamers ; but the 
extraordinary differences of intensity between the 
inner corona and its outlying extensions are such 
that halation renders a complete picture on a single 
plate practically impossible. The filamentous detail 
of the inner corona, and the faintest outlying ex- 
tensions or streamers, the eye must still reveal di- 
rectly. 

In solar spectrum photography, research has been 
especially benefitted; indeed, exact registry of the 
multitudinous lines was quite impossible without it. 
Photographic maps of the spectrum by Thollon, 
McClean and Rowland are so complete and accurate 
that no visual charts can approach them. Rowland's 
great photographic map of the solar spectrum 
spread out into a band about forty feet in length; 
and in the infra-red, Langley's speotrobolometer ex- 
tended the invisible heat spectrum photographically 
to many times that length. At the other end of the 
spectrum, special photographic processes have ex- 
tended the ultra-violet spectrum far beyond the 
ocular limit, to a point where it is abruptly cut off by 
absorption of the earth's atmosphere. On the same 
plate with certain regions of the sun's spectrum, the 
spectra of terrestrial metals are photographed side 
by side, and exact coincidences of lines show that 
about forty elemental substances known to terres- 
trial chemistry are vaporized in the sun. 

Young was the first to photograph a solar prom- 
inence in 1870, and twenty years later Deslandres 
of Paris and Hale of Chicago independently in- 




A. View of the 100-foot Dome in Which the Largest Telescope 
IN THE World is Housed. {Courtesy, Mt. Wilson Solar Observatory.) 




Mount Chimborazo, Xear the Equator. An observatory located on 
this mountain would make it possible to study the phenomena of 
northern and southern skies from the same point. (Courtesy, Pan- 
American Union.) 




LiPK Oe^ekva.oi>y. cn thz SuMair of Mr. Hamilton, Abcui 

Twenty-Five Miles S. W. of San Jose, California. It contains 

the famous Lick telescope, a 36-inch refractor. 




Near View of the Bye-End of the Yerkes Telescope. The eye- 
piece is removed and its place taken by a photographic plate. 



ASTRONOMICAL PHOTOGRAPHY 129 

vented the spectroheliograph, by which the chromo- 
sphere and prominences of the sun, as well as the 
disk of the sun itself, are all photographed by mono- 
chromatic light on a single plate. Hale has de- 
veloped this instrument almost to the limit, first 
at the Yerkes Observatory of the University of 
Chicago, and more recently at the Mount Wilson 
Observatory of the Carnegie Institution, where spec- 
troheliograms of marvelous perfection are daily 
taken. It was with this instrument that Hale dis- 
covered the effect of an electromagnetic field in sun 
spots which has revolutionized solar theories, a re- 
search impossible to conceive of without the aid of 
photography. 

When we apply Doppler's principle^ photography 
becomes doubly advantageous, whether we deter- 
mine, as Duner did and more recently Adams, the 
sun's own rotation and find it to vary in different 
solar latitudes, the equator going fastest; or apply 
the method to the sun^s corona at the east and west 
limbs of the sun, which Desiandres in 1893 
proved to be rotating bodily with the sun, because 
of the measured displacement of spectral lines 
of the corona in juxtaposition on the photo- 
graphic plate. 

In the solar astronomy of measurement, too, pho- 
tography has been helpfully utilized, as in register- 
ing the transits of Mercury over the sun's disk, for 
correcting the tables of the planet's orbital motion ; 
and most prominently in the action taken by the 
principal governments of the world in sending out 
expeditions to observe the transits of Venus in 1874 
and 1882, for the purpose of determining the paral- 
lax of Venus and so the distance of the earth from 
the sun. 

Scl. Vol. 2—5 



130 ASTRONOMY TO-DAY 

In our studies of the moon, photography has al- 
most completely superseded ocular work during the 
past sixty years. Rutherfurd and Draper of New 
York about 1865 obtained very excellent lunar pho- 
tographs with wet plates, which were unexcelled for 
nearly half a century. The Harvard, Lick, and Paris 
Observatories have published pretty complete photo- 
graphic atlases of the moon, and the best negatives 
of these series show nearly everything that the eye 
can discern, except under unusual circumstances. 
Later lunar photography was taken up at the Yerkes 
Observatory, and exceptionally fine photographs on 
a large scale were obtained with the 40-inch re- 
fractor, using a color screen. More recently the 
60-inch and 100-inch mirrors of the Mount Wilson 
Observatory have taken a series of photographs of 
the moon far surpassing everything previously done, 
as was to be expected from the unique combination 
of a tranquil mountain atmosphere with the ex- 
traordinary optical power of the instruments, and a 
special adaptation of photographic methods. Dur- 
ing lunar eclipses, Pickering has made a photo- 
graphic search for a possible satellite of the moon, 
occultations of stars by the moon have been recorded 
by photography, and Russell of Princeton has shown 
how the position of the moon among the stars can 
be determined by the aid of photography with a high 
order of precision. 

The story of planetary photography is on the 
whole disappointing. Much has been done, but there 
is much that is within reach, or ought to be, that 
remains undone. From Mercury nothing ought per- 
haps to be expected. On many of the photographs 
of the transit of Venus, especially those taken under 
the writer's direction at the Lick Observatory in 



ASTRONOMICAL PHOTOGRAPHY 131 

1882, we have unmistakable evidence of the planet's 
atmosphere. Here again the wet plate process, al- 
though more clumsy, demonstrated its superiority 
over the dry process used by other expeditions. 

In spectroscopy, Belopolsky has sought to deter- 
mine the period of rotation of Venus on her axis. At 
the Lowell Observatory, Douglass succeeded in pho- 
tographing the faint zodiacal light, and very success- 
ful photographs of Mars were taken at this institu- 
tion as early as 1905 by Slipher. Two years later 
these were much improved upon by the writer's ex- 
pedition to the Andes of Chile, when 12,000 expos- 
ures of Mars were made, many of them showing the 
principal canali, and other prominent features of 
the planet's disk. At subsequent oppositions of the 
planet, Barnard at the Yerkes Observatory and the 
Mount Wilson observers have far surpassed all these 
photographs. 

For future oppositions a more sensitive film is 
highly desired, in connection with instruments pos- 
sessing greater light-gathering power, so permitting 
a briefer exposure that will be less influenced by ir- 
regularities and defects of the atmosphere. The 
spectrum of Mars is of course that of sunlight, very 
much reduced, and modified to a slight extent by its 
passing twice through the atmosphere of Mars. 
What amount of aqueous vapor that atmosphere may 
contain is a question that can be answered only by 
critical comparison of the Martian spectrum with 
the spectrum of the moon, and photography affords 
the only method by which this can be done. 

Many are the ways in which photography has 
aided research on the asteroid group. Since 1891 
more than 600 of them have been discovered by pho- 
tography, and it is many times easier to find the 



132 ASTRONOMY TO-DAY 

new object on the photographic plate than to detect 
it in the sky as was formerly done by means of star 
charts. The planet by its motion during the ex- 
posure of the plate produces a trail, whereas the sur- 
rounding stars are ail round dots or images. Or by 
moving the plate slightly during exposure, as in 
Metcalf's ingenious method, we may catch the 
planet at that point where it will give a nearly 
circular image, and thus be quite as easy to detect, 
because all the stars on the same plate will then 
be trails. 

Photographic photometry of the asteroids has re- 
vealed marked variations in their light, due perhaps 
to irregularities of figure. On account of their faint 
light, the asteroids are especially suited, as Mars is 
not, to exact photography for ascertaining their 
parallax, and from this the sun's distance when the 
asteroid's distance has been found. Many asteroids 
have been utilized in this way, in particular Eros 
(433). In 1931 it approaches the earth within 
13 million miles, when the photographic method 
will doubtless give the sun's distance with the ut- 
most accuracy. 

Photographs of Jupiter have been very success- 
fully taken at the Yerkes and Lowell Observatories 
and elsewhere, but the great depth of the planet's 
atmosphere is highly absorptive, so that the im- 
pression is very weak in the neighborhood of the 
limb, if the exposure is correctly timed for the cen- 
ter of the disk. The striking detail of the belts, how- 
ever, is excellently shown. Wood of Baltimore has 
obtained excellent results by monochromatic photog- 
raphy of Jupiter and Saturn with the 60-inch re- 
flector on Mount Wilson. Jupiter's satellites have 
not been neglected photographically, and Pickering. 



ASTRONOMICAL PHOTOGRAPHY 133 

has observed hundreds of the eclipses of the satel- 
lites by a sort of cinematographic method of re- 
peated exposures, around the time of disappearance 
and reappearance by eclipse. The newest outer 
satellites of Jupiter were all discovered by photog- 
raphy, and it is extremely doubtful if they would 
have been found otherwise. 

Saturn has long been a favorite object with the 
astronomical photographer, and there are many 
fine pictures in spite of his yellowish light, rela- 
tively weak photographically. The marvelous ring 
system with the Cassini division, the oblateness of 
the ball^ the occasional markings on it — all are well 
shown in the best photographs; but the call is for 
more light and a more sensitive photographic proc- 
ess. Pickering's ninth satellite (Phoebe) was dis- 
covered by photography, one of the faintest moons 
in the solar system. Like the faint outer moons 
of Jupiter, few existing telescopes are powerful 
enough to show it. Its orbit has been found from 
photographic observations, and its position is 
checked up from time to time by photography. 

But the crowning achievement of spectrum pho- 
tography in the Saturnian system is Keeler's appli- 
cation of Doppler's principle in determining the 
rate of orbital motion of particles in different zones 
of the rings, thereby establishing the Maxwellian 
theory of the constitution of the rings beyond the 
possibility of doubt. For Uranus and Neptune 
photography has availed but little, except to nega- 
tive the existence of additional satellites of these 
planets, which doubtless would have been discovered 
by the thorough photographic search which has 
been made for them by W. H. Pickering without 
result. 



134 ASTRONOMY TO-DAY 

As with the asteroids, so with comets : several of 
these bodies have been discovered by photography; 
none more spectacular than the Egyptian comet of 
May 17th, 1882, which impressed itself on the plates 
of the corona of that date. Withdrawal of the sun's 
light by total eclipse made the comet visible, and it 
had never been seen before, nor is it known whether 
it will ever return. In comet ary photography, much 
the same difficulties are present as in photographing 
the corona: if the plate is exposed long enough to 
get the faint extensions of the tail, the fine filaments 
of the coma or head are obliterated by halation and 
overexposure. 

No one has had greater success in this work than 
Barnard, whose photographs of comets, particu- 
larly at the Lick Observatory, are numerous and un^ 
excelled. His photographs of the Brooks Comet of 
1893 revealed rapid and violent changes in the tail, 
as if shattered by encounter with meteors ; and the 
tail of Halley's comet in 1910 showed the rapid pro- 
pagation of luminous waves down the tail, similar to 
phenomena sometimes seen in streamers of the 
aurora. Draper obtained the first photograph of a 
comet's spectrum in 1881, disclosing an identity 
with hydrocarbons burning in a Bunsen flame, also 
bands in the violet due to carbon compounds. The 
photographic spectra of subsequent comets have 
shown bright lines due to sodium and the vapor of 
iron and magnesium. 

Even the elusive meteor has been caught by 
photography, first by Wolf in 1891, who was ex- 
posing a plate on stars in the Milky Way. On de- 
veloping it, he found a fine, dark nearly uniform 
line crossing it, due to the accidental flight across 
the field of a meteor of varying brightness. Since 



ASTRONOMICAL PHOTOGRAPHY 135 

then meteor trails have been repeatedly photo- 
graphed, and even the trail spectra of meteors have 
been registered on the Harvard plates. At Yale 
in 1894 Elkin employed a unique apparatus for 
securing photographic trails of meteors : six photo- 
graphic cameras mounted at different angles on a 
long polar axis driven by clockwork, the whole 
arranged so as to cover a large area of the sky where 
meteors were expected. 

When we pass from the solar system to the stellar 
universe the advantages of photography and the am- 
plification of research due to its employment as 
accessory in nearly every line of investigation are 
enormous. So extensively has photography been 
introduced that plates, and to a slight extent films, 
are now almost exclusively used in securing original 
records. Regrettably so in case of the nebulae, be- 
cause the numerous photographs of the brighter 
nebtilse taken since 1880 when Draper got the first 
photograph of the nebula of Orion, are as a rule not 
comparable with each other. Differences of instru- 
ments, of plates, of exposure, and development — all 
have occasioned differences in portrayal of a nebula 
which do not exist. When we consider faithful 
accuracy of portrayal of the nebulae for purposes of 
critical comparison from age to age, many of our 
nebular photographs of the past forty years, fine 
as they are and marvelous as they are, must fail 
to serve the purpose of revealing progressive 
changes in nebular features in the future. 

Roberts and Common in England were among the 
fifst to obtain nebular photographs with extraordi- 
nary detail, also the brothers Henry of Paris. As 
early as 1888 Roberts revealed the true nature of 
the great nebula in Andromeda, which had never 



136 ASTRONOMY TO-DAY 

been suspected of being spiral; and Keeler and 
Perrine at the Lick Observatory pushed the photo- 
graphic discovery of spiral nebulae so far that their 
estimates fill the sky vdth many hundred thousands 
of these objects. 

In the southern hemisphere the 24-inch Bruce 
telescope of Harvard College Observatory has 
obtained many very remarkable photographs of 
nebulae, particularly in the vicinity of Eta Carinse. 
But the great reflectors of the Mount Wilson Observ- 
atory, on account of their exceptional location and 
extraordinary power, have surpassed all others in 
the photographic portrayal of these objects, especi- 
ally of the spiral nebula which appear to show all 
stages in transition from nebula to star. No less 
remarkable are the photographs of such wonderful 
clusters as Omega Centauri, a perfect visual repre- 
sentation of which is wholly impossible. Intercom- 
parison of the photographs of clusters has afforded 
Baily of Harvard, Shapley of Mount Wilson and 
others the opportunity of discovery that hundreds 
of the component stars are variable. 

What is the longest photographic exposure ever 
made? At the Cape of Good Hope, under the direc- 
tion of the late Sir David Gill, exposures on nebulae 
were made, utilizing the best part of several nights, 
and totaling as high as seventeen, or even twenty- 
three hours. But the Mount Wilson observers have 
far surpassed this duration. To study the rotation 
and radial velocity of the central part of the nebula 
of Andromeda, an exposure of no less than 79 hours* 
total duration was made on the exceedingly faint 
spectrum, and even that record has since been 
exceeded. The eye cannot be removed from the 
guiding star for a moment while the exposure 



ASTRONOMICAL PHOTOGRAPHY 137 

is in progress, and this tedious piece of work was 
rewarded by determining the velocity of the center 
of the nucleus as a motion of approach at the rate 
of 316 kilometers per second. 

But when the stars, their magnitudes and their 
special peculiarities are to be investigated en masse, 
photography provides the facile means for re- 
searches that would scarcely have been dreamed of 
without it. The international photographic chart of 
the entire heavens, in progress at twenty observa- 
tories since 1887, the photographic charts of the 
northern heavens at Harvard and of the southern sky 
at Cape Town, the manifold investigations that have 
led up to the Harvard photometry, and the unpar- 
alleled photographic researches of the Henry Draper 
Memorial, enabling the spectra of many hundred 
thousand stars to be examined and classified — all 
this is but a part of the astronomical work in stellar 
fields that photography has rendered possible. 

Then there are the stellar parallaxes, now ob- 
served for many stars at once photographically, 
when formerly only one star's parallax could be 
measured at a time and with the eye at the telescope. 
And photo-electric photometry, measuring smaller 
differences of light than any other method, and pro- 
viding more accurate light-curves of the variable 
stars. And perhaps most remarkable of all, the 
radial velocity work on both stars and nebula, giv- 
ing us the distance of whole classes of stars, dis- 
covering large numbers of spectroscopic binaries 
and checking up the motion of the solar system 
toward Lyra within a fraction of a mile per 
second. 

All told, photography has been the most potent 
adjunct in astronomical research, and it is impos- 



138 ASTRONOMY TO-DAY 

sible to predict the future with more powerful 
apparatus and photographic processes of higher 
sensitiveness. The field of research is almost 
boundless, and the possibilities practically without 
limit. 

What would Herschel have done with £100,000— 
and photography! 



CHAPTER XXII 

MOUNTAIN OBSERVATORIES 

THE century that has elapsed since the time of 
Sir William Herschel, known as the father of 
the new or descriptive astronomy, has witnessed all 
the advances of the science that have been made 
possible by adopting the photographic method of 
making the record, instead of depending upon the 
human eye. Only one eye can be looking at the 
eyepiece at a time: the photograph can be studied 
by a thousand eyes. 

At mountain elevations telescopes are now ex- 
tensively employed, and there the camera is of 
especial and additional value, because the photo- 
graph taken on the mountain can be brought down 
for the expert to study, at ease and in the comfort 
of a lower elevation. We shall next trace the move- 
ment that has led the astronomer to seek the sum- 
mits of mountains for his observatories, and the 
photographer to follow him. 

Not only did the genius of Newton discover the 
law of universal gravitation^ and make the first ex- 
periments in optics essential to the invention of the 
spectroscope, but he was the real originator also of 
the modern movement for the occupation of moun- 
tain elevations for astronomical observatories. His 
keen mind followed a ray of light all the way from 
its celestial source to the eye of the observer, and 
analyzed the causes of indistinct and imperfect 
vision. 

139 



140 ASTRONOMY TO-DAY 

Endeavoring to improve on the telescope as 
Galileo and his followers had left it, he found such 
inherent difficulties in glass itself that he abandoned 
the refracting type of telescope for the reflector, to 
the construction of which he devoted many years. 
But he soon found out, what every astronomer and 
optician knew to their keen regret, that a telescope, 
no matter how perfectly the skill of the optician's 
hand may make it, cannot perform perfectly unless 
it has an optically perfect atmosphere to look 
through. 

So Newton conceived the idea of a mountain 
observatory, on the summit of which, as he thought, 
the air would be not only cloudless, but so steady 
and equable that the rays of light from the heavenly 
bodies might reach the eye undisturbed by atmos- 
pheric tremors and quiverings which are almost 
always present in the lower strata of the great 
ocean of air that surrounds our planet. 

This is the way Newton puts the question in his 
treatise on Opticks — he says : . ''The Air through 
which we look upon the Stars, is in a perpetual 
Tremor; as may be seen by the tremulous Motion 
of Shadows cast from high Towers, and by the 

twinkling of the fix'd stars The only remedy 

is a most serene and quiet Air, such as may perhaps 
be found on the tops of the highest Mountains above 
the grosser Clouds." 

Newton's suggestion is that the highest moun- 
tains may afford the best conditions for tranquillity ; 
and it is an interesting coincidence that the summits 
of the highest mountains, about 30,000 feet in eleva- 
tion, are at about the same level where the turbu- 
lence of the atmosphere most likely ceases, according 
to the indications of recent meteorological research. 



MOUNTAIN OBSERVATORIES 141 

These heights are far above any elevations per- 
manently occupied as yet, but a good beginning has 
been made and results of great value have already 
been reached. 

Curiously, investigation of mountain peaks and 
their suitability for this purpose was not under- 
taken till nearly two centuries after Newton, when 
Piazzi Smyth in 1856 organized his expedition to 
the summit of a mountain of quite moderate eleva- 
tion, and published his "Teneriffe : an Astronomer's 
Experiment." Teneriffe is an accessible peak of 
about 10,000 feet, on an island of the Canaries off 
the African coast, where Smjrth fancied that condi- 
tions of equability would exist; and on reaching 
the summit with his apparatus and spending a few 
days and nights there, he was not disappointed. 
Could he have reached an elevation of 13,000 feet, 
he would have had fully one-third of all the atmos- 
phere in weight below him, and that the most tur- 
bulent portion of all. Nevertheless, the gain in 
steadiness of the atmosphere, providing ^'better see- 
ing," as the astronomer's expression is, even at 
10,000 feet, was most encouraging, and led to at- 
tempts on other peaks by other astronomers, a few 
of whom we shall mention. 

Davidson, an observer of the United States Coast 
Survey, with a broad experience of many years in 
mountain observing, investigated the summit of the 
Sierra Nevada mountains as early as 1872, at an 
elevation of 7,200 feet. His especial object was to 
make an accurate comparison between elevated sta- 
tions at different heights. He found the seeing 
excellent, especially on the sun; but the excessive 
snowfall at his station, 45 feet annually, was a con- 
dition very adverse to permanent occupation. 



142 ASTRONOMY TO-DAY 

In the summer of 1872, Young spent several 
weeks at Sherman, Wyoming, at an elevation ex- 
ceeding 8,300 feet. He carried with him the 9.4- 
inch telescope of Dartmouth College, where he was 
then professor, and this was the first expedition on 
which a large glass was used by a very skillful 
observer at great elevation. He found the number 
of good days and nights small, but the sky was ex- 
ceedingly favorable when clear. Many 7th magni- 
tude stars could be detected with the naked eye. 
Young's observations at Sherman were mainly 
spectroscopic, however, and they demonstrated the 
immense advantage of a high-level station, far above 
the dust and haze of the lower atmosphere. He 
pronounced the 9.4-inch glass at 8,000 feet the full 
equivalent of a 12-inch at sea level. 

Mont Blanc of 15,000 feet elevation was another 
summit where the veteran Janssen of Paris main- 
tained a station for many years ; but the continental 
conditions of atmospheric moisture and circulation 
were not favorable on the whole. Janssen was 
mainly interested in the sun, and the daylight seeing 
is rarely benefited, owing to the strong upward 
currents of warm air set in motion by the sun itself. 

Mountains in the beautiful climate of California 
were among the earliest investigated, and when in 
1874 the trustees of Mr. James Lick's estate were 
charged with equipping an observatory with the 
most powerful telescope in existence, they wisely 
located on the summit of Mount Hamilton. It is 
4,300 feet above sea level, and Burnham and other 
astronomers made critical tests of the steadiness of 
vision there by observing double stars, which afford 
perhaps the best means of comparing the optical 
quality of the atmosphere of one region with an- 



MOUNTAIN OBSERVATORIES 143 

other. The writer was fortunate in having charge 
of the observations of the transit of Venus in 1882 
on the mountain, when the Observatory was in 
process of construction, and the quahty of the photo- 
graphs obtained on that occasion demonstrated anew 
the excellence of the site. Particularly at night, for 
about nine months of the year, the seeing is ex- 
ceptionally good, especially when fog banks rolling 
in from the Pacific, cover the valleys below like a 
blanket, preventing harmful radiation from the soil 
below. 

The great telescope mounted in 1888, a 364nch 
refractor by Alvan Clark, has fulfilled every ex- 
pectation of its projectors, and justified the selection 
of the site in every particular. The elevation, al- 
though moderate, is still high enough to secure very 
marked advantage in clearness and steadiness of 
the air, and at the same time not so high that the 
health and activities of the observers are appreci- 
ably affected by the thinner air of the summit. This 
telescope is known the world over for the monu- 
mental contributions to science made by the able 
astronomers who have worked with it : among them 
Barnard who discovered the fifth satellite of Jupiter 
in 1892; Burnham, Hussey, and Aitken, who have 
discovered and measured thousands of close double 
stars ; Keeler, who spent many faithful years on the 
summit; and Campbell, the present director, whose 
spectroscopic researches on stellar movements have 
added greatly to our knowledge of the structure of 
the universe. Among the many lines of research 
now in progress at the Lick Observatory and in the 
D. 0. Mills Observatory at Santiago, Chile, are the 
discoveries of stars whose velocities in space are not 
constant, but variable with the spectral type of the 



144 ASTRONOMY TO-DAY 

star. Mr. Lick's bequest for the Observatory was 
about $700,000. So ably has this scientific trust 
been administered that he might well have endowed 
it with his entire estate, exceeding $4,000,000. 

Another California mountain that was early in- 
vestigated is Mount Whitney. Its summit eleva- 
tion is nearly 15,000 feet, and in 1881 Langley made 
its ascent for the purpose of measuring the solar 
constant. He found conditions much more favor- 
able than on Mount Etna, Sicily — elevation about 
10,000 feet — which he had visited the year before. 
But the height of Mount Whitney was such as to 
occasion him much inconvenience from mountain 
sickness, an ailment which is most distressing and 
due partly to lack of oxygen and partly to mere 
diminution of mechanical pressure. Mount Whitney 
was also visited many years after by Campbell for 
investigating the spectrum of Mars in comparison 
with that of the moon. Langley found on Mount 
Whitney an excellent station lower down, at about 
12,000 feet elevation ; and by equipping the two sta- 
tions with like apparatus for measuring the solar 
heat, he obtained very important data on the selec- 
tive absorption of the atmosphere. 

Returning from the transit of Venus in 1882, 
Copeland of Edinburgh visited several sites in the 
Andes of Peru, ascending on the railway from Mol- 
lendo. Vincocaya was one of the highest, something 
over 14,000 feet elevation. His report was most 
enthusiastic, not only as to clearness and transpar- 
ency of the atmosphere, but also as to its steadiness, 
which for planetary and double star observations 
is almost as important. Copeiand's investigation 
of this region of the Andes has led many other 
astronomers to make critical tests in the same 



MOUNTAIN OBSERVATORIES 145 

general region. Climatic conditions are particu- 
larly favorable, and the sites for high-level research 
are among the best known, the atmosphere being 
not only clear a large part of the year, but in certain 
favored spots exceedingly steady. 

In 1887 the writer ascended the summit of 
Fujiyama, Japan, 12,400 feet elevation. The early 
September conditions as to steadiness of atmosphere 
were extraordinarily fine, but the mountain is 
covered by cloud many months in each year. There 
is a saddle on the inside of the crater that would 
form an ideal location for a high-level observatory. 
This expedition was undertaken at the request of the 
late Professor Pickering, director of Harvard Col- 
lege Observatory, which had recently received a be- 
quest from Uriah A. Boyden, amounting to nearly a 
quarter of a million dollars, to "establish and main- 
tain, in conjunction with others, an astronomical 
observatory on some mountain peak." 

Great elevations were systematically investigated 
in Colorado and California, the Chilean desert of 
Atacama was visited, and a temporary station es- 
tablished at Chosica, Peru, elevation about 5,000 
feet. Atmospheric conditions becoming unfavorable, 
a permanent station was established in 1891 at 
Arequipa, Peru, elevation 8,000 feet, which has been 
maintained as an annex to the Harvard Observ- 
atory ever since. The cloud conditions have been 
on the whole less favorable than was expected, but 
the steadiness of the air has been very satisfactory. 
In addition to planetary researches conducted there 
in the earlier years by W. H. Pickering, many large 
programs of stellar research have been executed, 
especially relating to the magnitudes and spectra of 
the stars. In conjunction with the home pbserva- 



146 ASTRONOMY TO-DAY 

tory in the northern hemisphere, this afforded a 
vast advantage in embracing all the stars of the 
entire heavens, on a scale not attempted elsewhere. 
The Bruce photographic telescope of 24-inch aper- 
ture has been employed for many years at Arequipa, 
and with it the plates were taken which enabled 
Pickering to discover the ninth satellite of Saturn 
(PhcBbe), and the splendid photographs of southern 
globular clusters in which Bailey has found 
numerous variable stars of very short periods — 
very faint objects, but none the less interesting, and 
of much significance in modern study of the evolu- 
tion and structure of the stellar universe. The 
crowning research of the observatory is the Henry 
Draper catalogue of stellar spectra, now in process 
of publication, which is of the first order of impor- 
tance in statistical studies of stellar distribution 
with reference to spectral type, and in studying the 
relation of parallax and distance, proper motion, 
radial velocity and its variation to the spectral 
characteristics of the stars. 

Perrine of Cordova is now establishing on Sierra 
Chica about twenty-five miles southwest of Cor- 
dova, a great reflecting telescope comparable in size 
with the instruments of the northern hemisphere, 
for investigation of the southern nebulae and 
clusters, and motions of the stars. The elevation 
of this new Argentine observatory will be 4,000 feet 
above sea level. 

Another observatory at mountain elevation and 
in a highly favorable climate is the Lowell Obser- 
vatory, located at about 7,000 feet elevation at Flag- 
staff, Arizona. Many localities were visited and 
the atmosphere tested especially for steadiness, an 
optical quality yery essential for research on the 



MOUNTAIN OBSERVATORIES 147 

planetary surfaces. Mexico was one of these sta- 
tions, but local air currents and changes of temper- 
ature there were such that good seeing was far from 
prevalent, as had been expected. At Flagstaff, on 
the other hand, conditions have been pretty uni- 
formly good, and an enormous amount of work on 
the planet Mars has been accumulated and pub- 
lished. The first successful photographs of this 
planet were taken there in 1905, and Jupiter, 
Saturn, the zodiacal light and many other test 
objects have been photographed, which demon- 
strates the excellence of the site for astronomical 
research. Within recent years spectrum research 
by Slipher, especially on the nebulse, has been added 
to the program, and the rotation and radial veloci- 
ties of many nebulas have been determined. 

On Mount Wilson, near Pasadena, California, at 
an elevation of nearly 6,000 feet, is the Carnegie 
Solar Observatory, founded and equipped under the 
direction of Professor George E. Hale, as a depart- 
ment of the Carnegie Institution of Washington, of 
which Dr. Robert S. Woodward is President. The 
climatology of the region was carefully investigated 
and tests of the seeing made by Hussey and others. 
Although equipped primarily for study of the sun, 
the program of the observatory has been widely 
amplified to include the stars and nebulse. The in- 
strumental equipment is unique in many respects. 
To avoid the harmful effect of unsteadiness of air 
strata close to the ground a tower 75 feet high was 
erected, with a dome surmounting it and covering 
a coelostat with mirror for reflecting the sun's rays 
vertically downward. Underneath the tower a dry 
well was excavated to a depth equaling the height of 
the tower above it. In the subterranean chamber 



148 ASTRONOMY TO-DAY 

is the spectroheliograph of exceptional size and 
power. The sun's original image is nearly 17 inches 
in diameter on the plate, and the solar chromosphere 
and prominences, together with the photosphere and 
f aculse, are all recorded by monochromatic light. 

Connected with the observatory on Mount Wilson 
are the laboratories, offices and instrument shops in 
Pasadena, 16 miles distant, where the remarkable 
apparatus for use on the mountain is constructed. 
A reflecting telescope with silver-on-glass mirror 
60 inches in diameter was first built by Ritchey and 
thoroughly tested by stellar photographs. Also the 
northern spiral nebulae were photographed, exhibit- 
ing an extraordinary wealth of detail in apparent 
star formation. The success of this instrument 
paved the way for one similar in design, but with a 
mirror 100 inches in diameter, provided by gift of 
the late John D. Hooker of Los Angeles. The tele- 
scope was completed in 1919. Notwithstanding its 
huge size and enormous weight, the mounting is 
very successful, as well as the mirror. Mercurial 
bearings counterbalance the v/eight of the polar 
axis in large part. This great telescope, by far the 
largest and most powerful ever constructed, is now 
employed on a program of research in which its 
vast light-gathering power will be utilized to the 
full. Under the skillful management of Hale and 
his enthusiastic and capable colleagues, the confines 
of the stellar heavens will be enormously extended, 
and secrets of evolution of the universe and of its 
structure no doubt revealed. 

In all the mountain stations hitherto established, 
as the Lick Observatory at 4,000 feet, the Mount 
Wilson Observatory 6,000 feet, the Lowell Observa- 
tory at 7,000 feet, the Harvard Observatory at 8,000 



MOUNTAIN OBSERVATORIES 149 

feet; and Teneriffe and Etna at 10,000, Fujiyama 
at 12,000, Pike's Peak at 14,000, Mont Blanc and 
Mount Whitney at 15,000, the researches that have 
been carried on have fully demonstrated the vast 
advantage of increased elevation in localities where 
climatological conditions as well as elevation are 
favorable. Nevertheless, only one-half of the ex- 
treme altitude contemplated by Sir Isaac Newton 
has yet been attained. 

Can the greater heights be reached and perma- 
nently occupied ? Geographically and astronomically 
the most favorably located mountain for a great 
observatory is Mount Chimborazo in Ecuador. Its 
elevation is 22,000 feet, and it was ascended by Ed- 
ward Whymper in 1880. Situated very nearly on 
the earth's equator, almost the entire sidereal 
heavens are visible from this single station, and all 
the planets are favored by circumzenith conditions 
when passing the meridian. No other mountain in 
the world approaches Chimborazo in this respect. 
But the summit is perpetually snow-capped, ex- 
ceedingly inaccessible, and the defect of barometric 
pressure would make life impossible up there in the 
open. 

Only one method of occupation appears to be fea- 
sible. The permanent snow line is at about 16,000 
feet, where excellent water power is available. By 
tunneling into the mountain at this point, and diag- 
onally upward to the summit, permanent occupa- 
tion could be accomplished, at a cost not to exceed 
one million dollars. 

The rooms of the summit observatory would need 
to be built as steel caissons, and supplied with com- 
pressed air at sea-level tension. The practicability 
of this plan was demonstrated by the writer in 



150 ASTRONOMY TO-DAY 

September, 1907, at Cerro de Pasco, Peru. A steel 
caisson was carried up to an elevation exceeding 
14,000 feet. Patients suffering acutely with moun- 
tain sickness were placed inside this caisson, and on 
restoring the atmospheric pressure within it artifici- 
ally all unfavorable symptoms — headache, high res- 
piration and accelerated pulse — disappeared. There 
was every indication that if persons liable to this 
uncomfortable complaint were brought up to this 
elevation, or indeed any attainable elevation, under 
unreduced pressure, the symptoms of mountain 
sickness would be unknown. Comfortable occupa- 
tion of the highest mountain summits was thereby 
assured. 

The working of astronomical instruments from 
within air-tight compartments does not present any 
insurmountable difficulties, either mechanical or 
physical. Since the time these experiments were 
made, the Guayaquil-Quito railway has been con- 
structed over a saddle of Chimborazo, at an eleva- 
tion of 12,000 feet; and only six miles of railway 
would need to be built from this station to the point 
where the tunnel would enter the mountain. 

Only by the execution of some such plan as this 
can astronomers hope to overcome the baleful effects 
of an ever mobile atmosphere, and secure the ad- 
vantages contemplated by Sir Isaac Newton in that 
tranquillity of atmosphere, which he conceived as 
perpetually surrounding the summits of the highest 
mountains. 

In Russell's theory of the progressive development 
of the stars, from the giant class to the dwarf, an 
element of verification from observation is lacking, 
because hitherto no certain method of measuring 
the very minute angular diameters of the stars has 



MOUNTAIN OBSERVATORIES 151 

been successfully applied. The apparent surface 
brightness corresponding to each spectral type is 
pretty well known, and by dividing it into the total 
apparent brightness, we have the angular area sub- 
tended by the star, quite independent of the star's 
distance. This makes it easy to estimate the angu- 
lar diameter of a star, and Betelgeuse is the one 
which has the greatest angular diameter of all , 
whose distances v/e know. Antares is next in order 
of angular diameter, 0".043, Aldebaran 0".022, 
Arcturus 0".020, Pollux 0".013, and Sirius only 
0".007. 

Can these theoretical estimates be verified by 
observation? Clearly it is of the utmost impor- 
tance and the exceedingly difficult inquiry has been 
undertaken with the 100-inch reflector on Mount 
Wilson, employing the method of the interferometer 
developed by Michelson and described later on, an 
instrument undoubtedly capable of measuring much 
smaller angles than can be measured by any other 
known method. Unquestionably the interference 
of atmospheric waves, or in other words what as- 
tronomers call "poor seeing," will ultimately set the 
limit to what can be accomplished. "But even if/* 
says Eddington, "we have to send special expedi- 
tions to the top of one of the highest mountains in 
the world, the attack on this far-reaching problem 
must not be allowed to languish.*' 



CHAPTER XXIII 

THE PROGRAM OF A GREAT OBSERVATORY 

THE Mount Wilson Observatory has now been in 
operation about fifteen years. The novelty in 
construction of its instruments, the investigations 
undertaken with them and the discoveries made, the 
interpretation of celestial phenomena by laboratory 
experiment, and the recent addition to its equipment 
of a telescope 100 inches in diameter, surpassing all 
others in power, directs especial attention to the ex- 
tensive activities of this institution, whose budget 
now exceeds a million dollars annually. Results are 
only achieved by a carefully elaborated program, 
such as the following, for which the reader is mainly 
indebted to Dr. Hale, the director of the observa- 
tory, who gives a very clear idea of the trend of 
present-day research on the magnetic nature of the 
sun, and the structure and evolution of the sidereal 
universe. 

The purpose of the observatory, as defined at its 
inception, was to undertake a general study of 
stellar evolution, laying especial emphasis upon the 
study of the sun, considered as a typical star; 
physical researches on stars and nebulss; and the 
interpretation of solar and stellar phenomena by 
laboratory experiments. Recognizing that the de- 
velopment of new instruments and methods afforded 
the most promising means of progress, well- 
equipped machine shops and optical shops were pra- 
.vided with this end in view. 
' 152 



PROGRAM OF A GREAT OBSERVATORY 153 

The original program of the observatory has 
been much modified and extended by the independ- 
ent and striking discovery by Campbell and Kap- 
teyn of an important relationship between stellar 
speed and spectral type; the demonstration by 
Hertzsprung and Russell of the existence of giant 
and dwarf stars; the successful application of 
the 60-inch reflector by Van Maanen to the measure- 
ment of minute parallaxes of stars and nebulae ; the 
important developments of Shapley's investigation 
of globular star clusters ; the possibilities of research 
resulting from Seares's studies in stellar photom- 
etry; and the remarkable means of attack devel- 
oped by Adams through the method of spectroscopic 
parallaxes. 

By this method the absolute magnitude, and hence 
the distance of a star is accurately determined from 
estimates of the relative intensities of certain lines 
in stellar spectra. Attention was first directed to- 
ward lines of this character in 1906, v/hen it was 
inferred that the weakening of some lines in the 
spectra of sun spots and the strengthening of others 
was the result of reduced temperature of the spot 
vapors. On testing this hypothesis by laboratory 
experiments, it was fully verified. 

Subsequently Adams, who had thus become fa- 
miliar with these lines and their variability, studied 
them extensively in the spectra of other stars. In 
this way was discovered the dependence of their 
relative intensities on the star's absolute magnitude, 
so providing the powerful method of spectroscopic 
parallaxes. 

This method, giving the absolute magnitude as 
well as the distance of every star (excepting those 
of the earliest type) whose spectrum is photo- 



154 ASTRONOMY TO-DAY 

graphed, is no less important from the evolutional 
than from the structural point of view. 

Investigations in solar physics which formerly 
held chief place in the research program have 
developed along unexpected lines. It could not be 
foreseen at the outset that solar magnetic phenom- 
ena might become a subject of inquiry, demanding 
special instrumental facilities, and throwing light 
on the complex question of the nature of the sun 
spots and other solar problems of long standing. It 
is obvious that these researches, together with those 
on the solar rotation and the motions of the solar 
atmosphere, developed by Adams and St. John, must 
be carried to their logical conclusion, if they are to 
be utilized to the fullest in interpreting stellar and 
nebular phenomena. 

The discovery of solar magnetism, like many 
other Mount Wilson results, was the direct outcome 
of a long series of instrumental developments. The 
progressive improvement and advance in size of the 
tools of research was absolutely necessary. Hale's 
first spectroheliograph at Kenwood in 1890 was at- 
tached to a 12-inch refractor, and the solar image 
was but two inches in diameter. It was soon found 
that a larger solar image was essential, and a spec- 
trograph of much greater linear dispersion ; in fact, 
the spectrograph must be made the prime element 
in the combination, and the telescope so designed 
as to serve as a necessary auxiliary. 

Accordingly, successive steps have led through 
spectrographs of 18 and 30 feet dimension to a 
vertical spectrograph 75 feet in focal length. The 
telescope is the 150 feet tower telescope, giving a 
solar image of 16.5 inches in diameter. Its spectro- 
graph is massive in construction, and by extending 



PROGRAM OF A GREAT OBSERVATORY 155 

deep into the earth, it enjoys the stability and con- 
stancy of temperature required for the most exact- 
ing work. 

Another direct outgrowth of the work of sun-spot 
spectra is a study of the spectra of red stars, where 
the chemistry of these coolest regions of the sun is 
partially duplicated. The combination of titanium 
and oxygen, and the significant changes of line in- 
tensity already observed in both instances, and also 
in the electric furnace at reduced temperatures, give 
indication of what may be expected to result from 
an attack on the spectra of the red stars with more 
powerful instrumental means, which is now pro- 
vided by the 100-inch telescope and its large stellar 
spectrograph. 

Other elements in the design of the 100-inch 
Hooker telescope have the same general object in 
view — that of developing and applying in astronom- 
ical practice the effective research methods sug- 
gested by recent advances in physics. Fresh possi- 
bilities of progress are constantly arising, and these 
are utilized as rapidly as circumstances permit. 

The policy of undertaking the interpretations of 
celestial phenomena by laboratory experiments, an 
important element in the initial organization of 
Mount Wilson, has certainly been justified by its 
results. Indeed, the development of many of the 
chief solar investigations would have been impos- 
sible without the aid of special laboratory studies, 
going hand in hand with the astronomical observa- 
tions. So indispensable are such researches, and so 
great is the promise of their extension, that the 
time has now come for advancing the laboratory 
work from an accessory feature to full equality with 
the major factors in the work of the observatory. 



156 ASTRONOMY TO-DAY 

Accordingly a new instrument now under installa- 
tion is an extremely powerful electro-magnet, de- 
signed by Anderson for the extension of researches 
on the Zeeman effect, and for other related investi- 
gations. Within the large and uniform field of this 
magnet, which is built in the form of a solenoid, a 
special electric furnace, designed for this purpose 
by King, is used for the study of the inverse Zee- 
man effect at various angles with the lines of force. 
This will provide the means of interpreting cer- 
tain remarkable anomalies in the magnetic phe- 
nomena of sun spots. 

The 100-inch telescope is now in regular use. All 
the tests so far applied show that it greatly sur- 
passes the 60-inch telescope in every class of work. 
For many months most of the observations and pho- 
tographs have been made with the Cassegrain com- 
bination of mirrors, giving an equivalent focal 
length of 134 feet and involving three reflections of 
light. The 100-inch telescope is found to give nearly 
2.8 times as much light as the 60-inch telescope, and 
therefore extends the scope of the instrument to all 
the stars an entire magnitude fainter. This is a 
very important gain for research on the faint 
globular clusters, as v/ell as the small and faint 
spiral and planetary nebulse, providing a much 
larger scale for these objects and sufficient light at 
the same time. Photographs of the moon and many 
other less critical tests have been made with very 
satisfactory results. Those of the moon appear to 
be decidedly superior in definition to any previously 
taken with other instruments. 

Another investigation is of great importance in 
the light of recent advances in theoretical dynamics. 
Darwin, in his fundamental researches on the dy- 



PROGRAM OF A GREAT OBSERVATORY 157 

namics of rotating masses, dealt with incompres- 
sible matter, which assumes the well-known pear- 
shaped figure, and may ultimately separate into two 
bodies. Roche on the other hand discussed the evo- 
lution of a highly compressible mass, which finally 
acquires a lens-shaped form and ejects a matter at 
its periphery. Both of these are extreme cases. 
Jeans has recently dealt with intermediate cases, 
such as are actually encountered in stars and 
nebulae. He finds that when the density is less 
than about one-fourth that of water, a lens-shaped 
figure will be produced with sharp edges, as de- 
picted by Roche. Matter thrown off at opposite 
points on the periphery, under the influence of small 
tidal forces from neighboring masses, may take the 
form of two symmetric filaments, though it is not 
yet entirely clear how these may attain the char- 
acteristic configuration of spiral nebulse. The pre- 
liminary results of Van Maanen indicate motion 
outward along the arms, in harmony with Jeans's 
views. 

Jeans further discusses the evolution of the arms, 
which will break up into nuclei (of the order of 
mass of the sun) if they are sufficiently massive, but 
will diffuse away if their gravitational attraction is 
small. The mass of our solar system is apparently 
not great enough, according to Jeans, to account for 
its formation in this way. As is apparent, these in- 
vestigations lead to conclusions very different from 
those derived by Chamberlin and Moulton from the 
planetesimal hypothesis. 

This is a critical study of spiral nebulae for which 
the 100-inch telescope is of all instruments in exis- 
tence the best suited. The spectra of the spirals 
must be studied, as well as the motions of the matter 



158 ASTRONOMY TO-DAY 

composing the arms. Their parallaxes, too, must be 
ascertained. A photographic campaign including 
spiral nebula of various types will settle the ques- 
tion of internal motions. The large scale of the 
spiral nebulas at the principal focus of the Hooker 
telescope, and the experience gained in the measure- 
ment of nebular nuclei for parallax determination, 
will help greatly in this research. A multiple-slit 
spectrograph, already applied at Mount Wilson, will 
be employed, not only on spiral nebulss whose plane 
is directed toward us, but also on those whose plane 
lies at an angle sufficient to permit both components 
of motion to be measured by the tv/o methods. 

In dealing with problems of structure and motion 
in the Galactic system, the 100-inch telescope offers 
especial advantages, because of its vast light-gather- 
ing power. Studies of radial velocities of the stars 
have hitherto been necessarily confined to the 
brighter stars, for the most part even to those visible 
to the naked eye. While some of these are very 
distant, most of the stars vfhose radial velocities are 
known belong to a very limited group, perhaps con- 
stituting a distinct cluster of v/hich the sun is a 
member, but in any event of insignificant propor- 
tions when contrasted with the Galaxy. Current 
spectrographic work with the 60-inch telescope in- 
cludes stars of the eighth magnitude, and some even 
fainter. But while the 60-inch has enabled Adams 
to measure the distances of many remote stars by 
his new spectroscopic method, and to double the 
known extent (so far as spectroscopic evidence is 
concerned) of the star streams of Kapteyn, a much 
greater advance into space is necessary to find out 
the community of motion among the stars compris- 
ing the Galactic system. The Hooker telescope will. 



PROGRAM OF A GREAT OBSERVATORY 159 

enable us to determine accurate radial velocities 
to stars of the eleventh magnitude, which doubtless 
truly represent the Galaxy. 

In order to secure a maximum return within a 
reasonable period of time, the stars in the selected 
areas of Kapteyn will be given the preference, be- 
cause of the vast amount of work already done, re- 
lating to their positions, proper motions, and visual 
and photographic magnitudes. Such consideration 
as spectral type, the known directions of star- 
streaming, and the position of the chosen regions 
with reference to the plane of the Galaxy are given 
adequate weight, and it is of fundamental impor- 
tance that the method of spectroscopic parallaxes 
will permit dwarf stars to be distinguished from 
stars that are in the giant class, but rendered faint 
by their much greater distance. In addition to these 
problems, the stellar spectrograms will provide rich 
material for study of the relationship between 
stellar mass and speed, and the nature of giant 
stars and dwarf stars. 

Shapley's recent studies of globular clusters have 
indicated the significance of these objects in both 
evolutional and structural problems, and the possi- 
bility of determining their parallaxes by a number 
of independent methods is of prime importance, both 
in its bearing on the structure of the universe and 
because it permits a host of apparent magnitudes to 
be at once transformed into absolute magnitudes. 
Here the advantage of the Hooker telescope is two- 
fold: at its 134-foot focus the increased scale of 
the crowded clusters makes it possible to select 
separate stars for spectrum photography (v/hich 
could not be done with the 60-inch where the images 
were commingled) ; and the great gain in light is 



160 ASTRONOMY TO-DAY 

such that the spectra of stars to the 14th magnitude 
have been photographed in loss than an hour. 

Faint globular clusters, then, will comprise a large 
part of the early program with the 100-inch tele- 
scope : the faintest possible stars in them must be de- 
tected and their magnitudes and colors measured; 
spectral types must be determined, and the radial 
velocities of individual stars and of clusters as a 
whole ; spectroscopic evidence of possible axial rota- 
tion of globular clusters must be searched for; and 
the method of spectroscopic parallaxes, as well as 
other methods, must be applied to ascertaining the 
distances of these clusters. 

The possibility of dealing with many problems 
relating to the distribution and evolution of the 
faintest stars depends upon the establishment of 
photographic and photovisual magnitude scales. 
Below the twelfth magnitude, the only existing scale 
of standard visual or photovisual magnitudes is the 
Mount Wilson sequence, already extended by Seares 
to magnitude 17.5 with the 60-inch telescope. 

Extension of this scale to even fainter magni- 
tudes, and its application to the faintest stars within 
its range is an important task for this great tele- 
scope, as it will doubtless bring within range hun- 
dreds of millions of stars that are beyond the reach 
of the 60-inch. The giants among them will form for 
us the outer boundary of the Galactic system, while 
the dwarfs will be of almost equal interest from the 
evolutional standpoint. The photometric program 
of the 100-inch, then, will deal with such questions 
as the condensation of the fainter stars toward the 
Galactic plane, the color of the most distant stars, 
and the final settlement of the long inquiry regard- 
ing the possible absorption of light in space. 



•' 


" # 








i §t^^^' ' 




*^ ^^m" 




* ^P' 




■• * 




The Sun's Disk. The view shows the "rice grain" structure of the 
photosphere and brilliant calcium flocculi. (PhotOj Yerkes Observatory.) 




The Lunar Surface Visible During a Total Eclipse of the Moon, 
February 8, 1906. (Photo, Yerkes Observatory.) 



PROGRAM OF A GREAT OBSERVATORY 161 

Another research of exceptional promise will be 
undertaken, which is of great importance in a gen- 
eral study of stellar evolution; and that is the de- 
termination of the spectral-energy curves of stars 
of various classes, for the purpose of measuring 
their surface temperatures. A very few of the 
nebulae are found to be variable, and their peculiar- 
ities need investigation, also special problems of 
variable stars and temporary stars, and the spectra 
of the components of close double stars which are 
beyond the power of all other instruments to 
photograph. 

Such a program of research conveys an excel- 
lent idea of many of the great problems that are 
under investigation by astronomers to-day, and gives 
some notion of the instrumental means requisite in 
executing comprehensive plans of this character. 
It will not escape notice that the climax of instru- 
mental development attained at Mount Wilson has 
only been made possible by an unbroken chain of 
progress, link by link, each antecedent link being 
necessary to the successful forging of its following 
one. In very large part, and certainly indispensable 
to these instrumental advances, has the art of work- 
ing in glass and metals been the mainstay of re- 
search. As we review the history of astronomical 
progress, from Galileo's time to our own, the con- 
summate genius of the artisan and his deft handi- 
work compel our admiration almost equally with the 
keen intelligence of the astronomer who uses these 
powerful engines of his own devising to wrest the 
secrets of nature from the heavens. 



Sci. Vol. 2—6 



CHAPTER XXIV 

OUR SOLAR SYSTEM 

NOW let us go upward in imagination, far, far be- 
yond the tops of the highest mountains, beyond 
the moon and sun, and outv/ard in space until we 
reach a point in the northern heavens millions and 
millions of miles away, directly above and equally 
distant from all points in the ecliptic, or path in 
which our earth travels yearly round the sun. Then 
we should have that sort of comprehensive view of 
the solar system which is necessary if we are to 
visualize as a whole the working of the vast machine, 
and the motions, sizes, and distances of all the bodies 
that comprise it. Of such stupendous mechanism our 
earth is part. 

Or in lieu of this, let us attempt to get in mind a 
picture of the solar system by means of Sir William 
Herschel's apt illustration : "Choose any well-leveled 
field. On it place a globe two feet in diameter. This 
will represent the sun ; Mercury will be represented 
by a grain of mustard seed on the circumference of 
a circle 164 feet in diameter for its orbit; Venus, 
a pea on a circle of 284 feet in diameter ; the Earth 
also a pea, on a circle of 430 feet; Mars a rather 
larger pin's head on a circle of 654 feet; the aster- 
oids, grains of sand in orbits of 1,000 to 1,200 feet; 
Jupiter, a moderate sized orange in a circle of nearly 
half a mile across; Saturn, a small orange on a 
circle of four-fifths of a mile; Uranus, a full-sized 

162 



OUR SOLAR SYSTEM 163 

cherry or small plum upon the circumference of a 
circle more than a mile and a half; and finally Nep- 
tune, a good-sized plum on a circle about two miles 
and a half in diameter. . . .To imitate the motions of 
the planets in the above mentioned orbits, Mercury 
must describe its own diameter in 41 seconds : Venus 
in 4 minutes, 14 seconds; the Earth in 7 minutes; 
Mars in 4 minutes 48 seconds ; Jupiter in 2 minutes 
56 seconds ; Saturn in 3 minutes 13 seconds ; Uranus 
in 2 minutes 16 seconds ; and Neptune in 3 minutes 
30 seconds." 

Now, let us look earthward from our imaginary 
station near the north pole of the ecliptic. All 
these planetary bodies would be seen to be traveling 
eastward round the sun, that is, in a counter-clock- 
wise direction, or contrary to the motions of the 
hands of a timepiece. Their orbits or paths of motion 
are very nearly circular, and the sun is practically 
at the center of all of them except Mercury and 
Mars ; of Venus and Neptune, almost at the absolute 
center. The planes of all their orbits are very nearly 
the same as that of the ecliptic, or plane in which 
the earth moves. These and many other resem- 
blances and characteristics suggest a uniformity of 
origin v/hich comports with the idea of a family, and 
so the whole is spoken of as the solar system, or 
the sun and his family of planets. 

In addition to the nine bodies already specified, 
the solar system comprises a great variety of other 
and lesser bodies ; no less than twenty-six moons or 
satellites tributary to the planets and traveling 
round them in various periods as the moon does 
round our earth. Then between the orbits of Mars 
and Jupiter are many thousands of asteroids, so 
called, or minor planets (about 1,000 of them have 



164 ASTRONOMY TO-DAY 

actually been discovered, and their paths accurately 
calculated). And at all sorts of angles with the 
planetary orbits are the paths of hundreds of comets, 
delicate filmy bodies of a wholly different constitu- 
tion from the planets, and which now and then blaze 
forth in the sky, their tails appearing much like the 
beam of a searchlight, and compelling for the time 
the attention of everybody. Connected with the 
comets and doubtless originally parts of them are 
uncounted millions of millions of meteors, which 
for the time become a part of the solar system, their 
minute masses being attracted to the planets, upon 
which they fall, those hitting the earth being visible 
to us as familiar shooting stars. 

We next follow the story of astronomy through 
the solar system, beginning with the sun itself and 
proceeding outward through his family of planets, 
now much more numerous and vastly more extended 
than it was to the ancient world, or indeed till 
within a century and a half of our own day. 



' CHAPTER XXV 

THE SUN AND OBSERVING IT 

AS lord of day, king of the heavens, mankind 
An the ancient world adored the sun. By their 
researches into the epoch of the Assyrians, Hittites, 
Phoenicians and other early peoples now passed 
from earth, archaeologists have unearthed many 
monuments that evidence the veneration in which 
the early peoples who inhabited Egypt and Asia 
Minor many thousand years ago held the sun. A 
striking example is found in the architecture of 
early Egyptian temples, on the lintels of which are 
carved representations of the winged globe or the 
winged solar disk, and there is a bare possibility that 
the wings of the globe were suggested by a type of 
the solar corona as glimpsed by the ancients. 

Little knew they about the distance and size of 
the sun ; but the effects of his light and heat upon all 
vegetal and animal life were obvious to them. Doubt- 
less this formed the basis for their worship of the 
sun. Occasional huge spots must have been visible 
to the naked eye, and the sun's corona was seen at 
rare intervals. Plutarch and Philostratus describe 
it very much as we see it today. 

How completely dependent mankind is upon the 
sun and its powerful radiations, only the science 
of the present day can tell us. By means of the sun's 
heat the forests of early geologic ages were enabled 
to wrest carbon from the atmosphere and store it in 

165 



166 ASTRONOMY TO-DAY 

forms later converted by nature's chemistry into 
peat and coal. Through processes but imperfectly 
understood, the varying forms of vegetable life are 
empowered to conserve, from air and soil, nitrogen 
and other substances suitable for and essential to the 
life maintenance of animal creatures. Breezes that 
bring rain and purify the air; the energy of water 
held under storage in stream and dam and fall; 
trade winds facilitating commerce between the con- 
tinents; oceanic currents modifying coastal cli- 
piates ; the violence of tornado, typhoon and water- 
spout, together with other manifestations of natural 
forces — all can be traced back to their origin in the 
tremendous heating power of the solar rays. In 
everything material the sun is our constant and 
bountiful benefactor. If his light and heat were 
withdrawn, practically every form of human activ- 
ity on this planet would come to an early end. 

How far away is the sun ? What is the size of the 
sun? These are questions that astronomers of the 
present day can answer with accuracy. 

So closely do they know the sun's distance that 
it is employed as their yardstick of the sky, or 
unit of celestial measurement. Many methods have 
been utilized in ascertaining the distance of the sun, 
and the remarkable agreement among them all is 
very extraordinary. Some of them depend upon 
pure geometry, and the basic measure which we 
make from the earth is not the distance of the sun 
directly; but we find out how far away Venus is 
during a transit of Venus, for example, or how far 
away Mars or some of the asteroids are at their 
closer oppositions. Then it is possible to calculate 
how far away the sun is, because one measurement 
of distance in the solar system affords us the scale 



THE SUN AND OBSERVING IT 167 

on which the whole structure is built. But perhaps 
the simplest method of getting the sun's distance is 
by the velocity of light, 186,800 miles a second. From 
eclipses of Jupiter's moons we know that light takes 
8 minutes 20 seconds to pass from sun to earth. 
So that the sun's distance is the simple product of 
the two, or 93 millions of miles. 

Once this fundamental unit is established, we have 
a firm basis on which to build up our knowledge of 
the distances, the sizes and motions of the heavenly 
bodies, especially those that comprise the solar sys- 
tem. We can at once ascertain the size of the sun, 
which we do by measuring the angle which it fills, 
that is, the sun's apparent diameter. Finding this 
to be something over a half a degree in arc> the proc- 
esses of elementary trigonometry tell us that the 
sun's globe is 865,000 miles in diameter. For nearly 
a century this has been accurately measured with 
the greatest care, and diameters taken in every 
direction are found to be equal and invariably the 
same. So we conclude that the sun is a perfect 
sphere, and so far as our instruments can inform 
us, its actual diameter is not subject to appreciable 
change. 

The vastness of the sun's volume commands our 
attention. As his diameter is 110 times that of the 
earth, his mere size or volume is 110x110, or 1,300 
thousand times that of the earth, because the 
volumes of spheres are in proportion as the cubes 
of their diameters. If the materials that compose 
the sun were as heavy as those that make up the 
earth, it would take 1,300 thousand earths to weigh 
as much as the sun does. But by a method which we 
need not detail here, the sun's actual weight or 
masa is found to be only 300 thousand (more nearly 



168 ASTRONOMY TO-DAY 

380,000) , times greater than the earth's. So we must 
infer that, bulk for bulk, the component materials 
of the sun are about one-fourth lighter than those 
of the earth, that is, about one and one-half times 
as dense as water. 

To look at this in another way: it is known that 
a body falling freely toward the earth from outer 
space would acquire a speed of seven miles a second, 
whereas if it were to fall toward the sun instead, 
the velocity would be 383 miles a second on reach- 
ing his surface. If all the other bodies of the solar 
system, that is, the earth and moon, all the planets 
and their satellites, the comets and all were to be 
fused together in a single globe, it would weigh only 
one-seven hundred and fiftieth as much as the sun 
does. 

At the surface, however, the disproportion of 
gravity is not so great, because of the sun's vast 
size : it is only about twenty-eight times greater on 
the sun than on the earth; and instead of a body 
falling 16 feet the first second as here, it would fall 
444 feet there. Pendulums of clocks on the sun 
would swing ^ve times for every tick here, and an 
athlete's running high jump would be scaled down 
to three inches. 

Let us next inquire into the amount of the sun's 
light and heat, and the enormously high temperature 
of a body whose heat is so intense even at the vast 
distance at which we are from it. The intensity 
of its brightness is such that we have no artificial 
source of light that we can readily compare it with. 
In the sky the next object in brightness is the full 
moon, but that gives less than the half-millionth 
part as much light as the sun. The standard candle 
used in physics gives so little light in comparison 



THE SUN AND OBSERVING IT 169 

that we have to use an enormous number to express 
the quantity of light that the sun gives. 

A sperm candle burning 120 grains hourly is the 
standard, and if we compare this with the sun when 
overhead, and allow for the light absorbed by the at- 
mosphere, we get the number 1575 with twenty-four 
ciphers following it, to express the candlepower of 
the sun's light. If we interpose the intense calcium 
light or an electric arc light between the eye and 
the sun, these artificial sources will look like black 
spots on the disk. Indeed, the sun is nearly four 
times brighter than the "crater,'' or brightest part 
of the electric arc. The late Professor Langley at 
a steel works in Pennsylvania once compared direct 
sunlight with the dazzling stream of molten metal 
from a Bessemer converter ; but bright as it was, sun- 
light was found to be five thousand times brighter. 

Equally enormous is the heat of the sun. Our in- 
tensest sources of artificial heat do not exceed 4,000 
degrees Fahrenheit, but the temperature at the sun's 
surface is probably not less than 16,000 degrees F. 
One square meter of his surface radiates enough 
heat to generate 100,000 horsepower continuously. 
At our vast distance of 93 millions of miles, the sun's 
heat received by the earth is still powerful enough to 
melt annually a layer of ice on the earth more than 
a hundred feet in thickness. If the solar heat that 
strikes the deck of a tropical steamship could be 
fully utilized in propelling it, the speed would reach 
at least ten knots. 

Many attempts have been made in tropical and 
sub-tropical climates to utilize the sun's heat directly 
for power, and Ericsson in Sweden, Mouchot in 
France, and Shuman in Egypt have built successful 
and efficient solar engines. Necessary intermission 



170 ASTRONOMY TO-DAY 

of their power at night, as well as on cloudy days, 
will preclude their industrial introduction until 
present fuels have advanced very greatly in cost. 
All regions of the sun's disk radiate heat uniformly, 
and the sun's own atmosphere absorbs so much that 
we should receive 1,7 times more heat if it were re- 
moved. So far as is known, solar light and heat are 
radiated equally in all directions, so that only a very 
minute fraction of the total amount ever reaches the 
earth, that is, 1 2200 millionth part of the whole. 
Indeed all the planets and other bodies of the solar 
system together receive only one one hundred mil- 
lionth part; the vast remainder is, so far as we 
know, effectively wasted. It is transformed, but 
what becomes of it, and whether it ever reappears in 
any other form, we cannot say. 

How is this inconceivably vast output of energy 
maintained practically invariable throughout the 
centuries? Many theories have been advanced, but 
only one has received nearly universal assent, that 
of secular contraction of the sun's huge mass upon 
itself. Shrinkage means evolution of heat ; and it is 
found by calculation that if the sun were to contract 
its diameter by shrinking only two-hundred and fifty 
feet per year, the entire output of solar heat might 
thus be accounted for. So distant is the sun and so 
slow this rate of contraction that centuries must 
elapse before we could verify the theory by actual 
measurements. Meanwhile, the progress of physical 
research on the structure and elemental properties 
of matter has brought to light the existence of 
highly active internal forces which are doubtless 
intimately concerned in the enormous output of 
radiant energy, though the mechanism of its main- 
tenance is as yet known only in part. 



THE SUN AND OBSERVING IT 171 

Abbot, from many years' observations of the 
solar constant, at Washington, on Mount Wilson, 
and in Algeria, finds certain evidence of fluctuation 
in the solar heat received by the earth. It cannot 
be a local phenomenon due to disturbances in our 
atmosphere, but must originate in causes entirely 
extraneous to the earth. Interposition of meteoric 
dust might conceivably account for it, but there is 
sufficient evidence to show that the changes must be 
attributed to the sun itself. The sun, then, is a vari- 
able star; and it has not only a period connected 
with the periodicity of the sun spots, but also an 
irregular, nonperiodic variation during a cycle of a 
week or ten days, though sometimes longer, and oc- 
casioning irregular fluctuations of two to ten pe]? 
cent of the total radiation. Radiation is found to 
increase with the spottedness. 

Attempts have been made on the basis of the con- 
traction theory to find out the past history of the sun 
and to predict its future. Probably 20 to 50 millions 
of years in the past represents the life of the sun 
much as it is at present; and if solar radiation in 
the future is maintained substantially as now, the 
sun will have shrunk to one-half its present diameter 
in the next five million years. 

So far then as heat and light from the sun are 
concerned, the sun may continue to support life on 
the earth not to exceed ten million years in the 
future. But the sun's own existence, independently 
of the orbs of the system dependent upon it, might 
continue for indefinite millions of seons before it 
would ever become a cold dead globe; indeed, in 
the present state of science, we cannot be sure 
that it is destined to reach that condition within 
calculable time. 



172 ASTRONOMY TO-DAY 

A few words on observing the sun, an object much 
neglected by amateurs. On account of the intense 
light, a very slight degree of optical power is suffi- 
cient. Indeed a piece of window glass, smoked in a 
candle flame with uniform graduation from end to 
end, will be found worth while in a beginner's daily 
observation of the sun. The glass should be smoked 
densely enough at one end so that the sunlight as 
seen through it will not dazzle the eye on the clearest 
days. At the other end of the glass, the degree of 
smoke film should not be quite so dense, so that 
the sun can be examined on hazy, foggy or partly 
cloudy days. An occasional naked-eye spot will re- 
ward the patient observer. 

If a small spyglass, opera glass or field glass is 
at hand, excellent views of the sun may be had by 
mounting the glass so that it can be held steadily 
pointed on the sun, and then viewing the disk by pro- 
jection on a white card or sheet of paper. Care 
must be taken to get a good focus on the projected 
image, and then the faculse, or whitish spots, or 
mottling nearer the sun's edge will usually be well 
seen. By moving the card farther away from the 
eyepiece, a larger disk may be obtained, in effect 
a higher degree of magnification. But care must be 
used not to increase it too much. Keep direct sun- 
light outside the tube from falling on the card where, 
the image is being examined. This is conveniently 
done by cutting a large hole, the size of the brass 
cell of the object glass, through a sheet of corrugated 
strawboard, and slipping this on over the cell. In 
this way the spots on the sun can be examined with 
ease and safety to the eye. 

For large instruments a special type of eyepiece 
is provided known as a helioscope, which disposes of 



THE SUN AND OBSERVING IT 173 

the intense heat rays that are harmful to the eye. 
Frequent examination of the eyepiece should be 
made and the eyepiece cooled if necessary. That 
part of the sun's surface under observation is known 
as the photosphere, that is, the part which radiates 
light. If the atmosphere admits the use of high 
magnifying powers, the structure of the photosphere 
will be found more and more interesting the higher 
the power employed. It is an irregularly mottled 
surface showing a species of rice-grain structure 
under fairly high magnification. These grains are 
grouped irregularly and are about 500 miles across. 
Under fine conditions of vision they may be sub- 
divided into granules. The faculse, or white spots, 
are sometimes elevations above the general solar 
level; they have occassionally been seen projecting 
outside the limb, or edge of the disk. 



CHAPTER XXVI 

SUN SPOTS AND PROMINENCES 

DARK spots of a deep bluish black will often be 
seen on the photosphere of the sun. Sometimes 
single, though generally in groups, the larger ones 
will have a dark center, called the umbra, sur- 
rounded by the very irregular penumbra which is 
darker near its outer edge and much brighter ap- 
parently on its inner edge where it joins on the 
umbra. The penumbra often shows a species of 
thatch-work structure, and systematic sketches of 
sun spots by observers skilled in drawing are greatly 
to be desired, because photography has not yet 
reached the stage where it is possible to compete 
with visual observation in the matter of fine detail. 
The spots themselves nearly always appear like de- 
pressions in the photosphere, and on repeated 
occasions they have been seen as actual notches 
when on the edge of the sun. 

Many spots, however, are not depressions: some 
appear to be actual elevations, with the umbra per- 
haps a central depression, like the crater in the 
general elevation of a volcano. Spots are some- 
times of enormous size. The largest on record was 
seen in 1858; it was nearly 150,000 miles in breadth, 
and covered a considerable proportion of the whole 
visible hemisphere of the sun. A spot must be 
nearly 30,000 miles across in order to be seen with 
the naked eye. 

174 



SUN SPOTS AND PROMINENCES 175 

In their beginning, development, and end, each 
spot or group of spots appears to be a law unto 
itself. Sometimes in a few hours they will form, 
though generally it is a question of days and even 
weeks. Very soon after their formation is com- 
plete, tonguelike encroachments of the penumbra 
appear to force their way across the umbra, and this 
splitting up of the central spot usually goes on quite 
rapidly. Sun spots in violent disturbances are rarely 
observed. As the sun turns round on his axis^ the 
spots will often be carried across the disk from the 
center to the edge, when they become very much 
foreshortened. The sun's period of rotation is 28 
days, so that if a spot lasts more than two weeks 
without breaking up, it may reappear on the eastern 
limb of the sun after having disappeared at the 
western edge. Two or three months is an average 
duration for a spot; the longest on record lasted 
through 18 months in 1840-41. 

The position of the sun's axis is well known, its 
equator being tilted about 7 degrees to the ecliptic, 
and the spots are distributed in zones north and 
south of the equator, extending as far as 30 degrees 
of solar latitude. In very high latitudes spots are 
never seen ; they are most abundant in about latitude 
15 degrees both north and south, and rather more 
numerous in the northern than in the southern 
hemisphere of the sun. Recent research at Mount 
Wilson makes the sun a great magnet; and its 
magnetic axis is inclined at an angle of 6 degrees 
to the axis of rotation, around which it revolves in 
32 days. 

There is a most interesting periodicity of the 
spots on the sun, for months will sometimes elapse 
with spots in abundance and visible every day, while 



176 ASTRONOMY TO-DAY 

at other periods, days and even weeks will elapse 
without a single spot being seen. There is a well 
recognized period of eleven and one-tenth years, the 
reason underlying which is not, however, known. 
After passing through the minimum of spottedness, 
they begin to break out again first in latitudes of 
25 degrees-30 degrees, rather suddenly, and on 
both sides of the equator, and they move toward 
the equator as their number and individual size 
decrease. 

The last observed epoch of maximum spot activity 
on the sun was passed in 1917. 

Many attempts have been made to ascertain the 
cause of the periodicity of sun spots, but the real 
cause is not yet known. If the spots are eruptional 
in character, the forces held in check during seasons 
of few spots may well break out in period. The 
brighter streaks and mottlings known as faculse are 
probably elevations above the general photosphere, 
and seem to be crusts of luminous matter, often in- 
candescent calcium, protruding through from the 
lower levels. Generally the faculse are numerous 
around the dark spots, and absorption of the sun's 
light by his own atmosphere affords a darker back- 
ground for them, with better visibility nearer the 
rim of the solar disk. The spectroheliograph re- 
veals vast zones of faculse otherwise invisible, 
related to the sun-spot zones proper on both sides 
of the equator. 

In some intimate way the magnetism of sun and 
earth are so related that outbreaks of solar spots 
are accompanied with disturbances of electrical and 
other instruments on the earth; also the aurora 
borealis is seen with greater frequency during 
periods when many spots are visible. 



SUN SPOTS AND PROMINENCES 177 

Within very recent years the discovery of a mag- 
netic field in sun spots has been made by Hale with 
powerful instruments of his own design. Sun spots 
had never been investigated before with adequate 
instrumental means. He recognized the necessity 
of having a spectroscope that would record the 
widened lines of sun-spot spectra, and the strength- 
ened and weakened lines on a large scale. Certain 
changes in relative intensity were traced to a re- 
duced temperature of the spot vapors by compari- 
son with photographs of the spectrum of iron and 
other metallic vapors in an electric arc at different 
temperatures. Here the work of the laboratory was 
essential. Sun spots were thus found to be regions 
of reduced temperature in the solar atmosphere. 
Chemical unions were thus possible, and thousands 
of faint lines in spot-spectra were measured and 
identified as band lines due to chemical compounds. 
Thus the chemical changes at work in sun-spot 
vapors were recognized. 

Then followed the highly significant investiga- 
tions of solar vortices and magnetic fields. Improve- 
ments in photographic methods had revealed im- 
mense vortices surrounding sun spots in the higher 
part of the hydrogen atmosphere ; and this led to the 
hypothesis that a sun spot is a solar storm, resem- 
bling a terrestrial tornado, and in which the hot 
vapors whirling at high velocity are cooled by ex- 
pansion. This would account for the observed inten- 
sity changes of the spectrum lines and the presence 
of chemical compounds. The vortex hypothesis sug- 
gested an explanation of the widening of many spot 
lines, and the doubling or trebling of some of them. 
As it is known that electrons are emitted by hot 
bodies, they must be present in vast numbers in the 



178 ASTRONOMY TO-DAY 

sun; and positive or negative electrons, if caught 
and whirled in a vortex, would produce a magnetic 
field. 

Zeeman in 1896 had discovered that the lines in 
the spectrum of a luminous vapor in a magnetic 
field are widened, or even split into several com- 
ponents if the field is strong enough. Characteristic 
effects of polarization appear also. The new ap- 
paratus of the observatory in conjunction with ex- 
periments in the laboratory immediately provided 
evidence that proved the existence of magnetic fields 
in sun spots, and strengthened the view that the 
spots are caused by electric vortices. 

Extended investigations have led Hale to the con- 
clusion that the sun itself is a magnet, with its poles 
situated at or near the poles of rotation. In this re- 
spect the sun resembles the earth, which has long 
been known to be a magnet. The sun's axial rota- 
tion permits investigation of the magnetic phe- 
nomena of all parts of its surface, so that ultimately 
the exact position of the sun's magnetic poles and the 
intensity of the field at different levels in the solar 
atmosphere will be ascertained. Schuster is of the 
opinion that not only the sun and earth, but every 
star, and perhaps every rotating body, becomes a 
magnet by virtue of its rotation. Hale is confident 
that the 100-inch reflector will permit the test for 
magnetism to be applied to a few of the stars. 

The sun can be observed at Mount Wilson on at 
least nine-tenths of all the days in the year, and a 
daily record of the polarities of all spots with the 
150-foot tower telescope is a part of the routine. A 
method has been devised for classifying sun spots on 
the basis of their magnetic properties, and more 
than a thousand spots have already been so classi- 



SUN SPOTS AND PROMINENCES 179 

fied. About 60 per cent of all sun spots are found 
to be binary groups, the single or multiple members 
of which are of opposite magnetic polarity. Uni- 
polar spots are very seldom observed without some 
indication of the characteristics of bipolar groups. 
These are usually exhibited in the form of flocculi 
following the spot. The bipolar spot seems to be 
the dominant type, and the unipolar type a variant 
of it. 

Although devised for quite another purpose, that 
of photographing the hydrogen prominences on the 
limb of the sun, the spectroheliograph has con- 
tributed very effectively to many departments of 
solar research. The prominences are dull reddish 
cloudlets that were first seen during total eclipses 
of the sun. Probably Vassenius, a Swedish astrono- 
mer, during the total eclipse of 1733, made the 
earliest record of them, as pinkish clouds quite de- 
tached from the edge of the moon ; and in that day, 
when it had not yet been proved that the moon was 
without atmosphere, he naturally thought they be- 
longed to the moon, not the sun. Undoubtedly Ulloa, 
a Spanish admiral, also saw the prominences in ob- 
serving the total eclipse of 1778 ; but they seem to 
have attracted little attention till 1842, when a very 
important total eclipse was central throughout 
Europe, and observed with great care by many of 
the eminent astronomers of all countries. 

So different did the prominences appear to dif- 
ferent eyes, and so many were the theories as to 
what they were, that no general consensus of opinion 
was reached, and some thought them no part of 
either sun or moon, but a mere mirage or optical 
illusion. But at the return of this eclipse in 1860, 
photography was employed so as to demonstrate be- 



180 ASTRONOMY TO-DAY 

yond a shadow of doubt the real existence and true 
solar character of the prominences. By the slow 
progress of the moon across the sun and the prom- 
inences on the edge, a unique series of photographs 
by De la Rue showed the moon's edge gradually 
cutting off the prominences piecemeal on one side 
of the sun, and equally gradually uncovering them 
on the opposite side. 

The prominences, then, were known to be real 
phenomena of the sun, some of them disconnectedly 
floating in his atmosphere, as if clouds. Their forms 
did not vary rapidly, they were very abundant, and 
their light was so rich in rays of great photographic 
intensity that many were caught on the plate which 
the eye failed to see; they appeared at every part 
of the sun's limb and their height above it indicated 
that they must be many thousand miles in actual 
dimension. What they were, however, remained an 
entire mystery, and no one even thought it possible 
to find out what their chemical constitution might 
be or to m^easure the speed with which they moved. 

A few years later came the great Indian eclipse 
(August 28, 1868), at that date the longest total 
eclipse ever observed. Janssen of France and many 
others went out to India to witness it. Fortunately 
the prominences were very brilliant and this led 
Janssen to believe it would be possible for him to 
see them the day after the eclipse was over. By 
modifying the adjustment of his apparatus suitably 
and changing its relation to the sun's edge, he found 
that hydrogen is the main constituent in the light of 
the prominences. In addition to this he was able 
to trace out the shapes of the prominences, and even 
measure their dimensions. His station in India was 
at Guntoor, many weeks by post from home ; so that 



SUN SPOTS AND PROMINENCES 181 

his account of this important discovery reached the 
Paris Academy of Sciences for communication with 
another from the late Sir Norman Lockyer of Eng- 
land, announcing a like discovery, wholly inde- 
pendently. 

The principle is simply this, and admirably stated 
by Young : "Under ordinary circumstances the prom- 
inences are invisible, for the same reason as the 
stars in the daytime : they are hidden by the intense 
light reflected from the particles of our own atmos- 
phere near the sun's place in the sky; and if we 
could only sufficiently weaken this aerial illumina- 
tion, without at the same time weakening their 
light, the end would be gained. And the spectro- 
scope accomplishes this very thing. Since the air- 
light is reflected sunshine, it of course presents the 
same spectrum as sunlight, a continuous band of 
color crossed by dark lines. Now, this sort of 
spectrum is greatly weakened by every increase of 
dispersive power, because the light is spread out into 
a longer ribbon and made to cover a more extended 
area. On the other hand, a spectrum of bright lines 
undergoes no such weakening by an increase in the 
dispersive power of the spectroscope. The bright 
lines are only more widely separated — not in the 
least difltused or shorn of their brightness." 

Simultaneous announcement of this great dis- 
covery, by astronomers of different nations, work- 
ing in widely separate regions of the earth, led to 
the striking of a gold medal by the French Govern- 
ment in honor of both astronomers and bearing their 
united effigies. Ever since the famous Indian eclipse 
of 1868, it has not been necessary to wait for a total 
eclipse in order to observe the solar prominences, 
but every observer provided with suitable apparatus 



182 ASTRONOMY TO-DAY 

has been able to observe them in full sunlight when- 
ever desired, and the charting of them is part of the 
daily routine at several observatories in different 
parts of the world. So vast has been the accumula- 
tion of data about them that we know their numbers 
to fluctuate with the spots on the sun ; and their dis- 
tribution over the sun*s surface resembles in a way 
that of the spots. 

While the spots and protuberances are most nu- 
merous around solar latitude 20 degrees both north 
and south, the prominences do not disappear above 
latitude 35 to 40 degrees, as the spots do, but from 
latitude 60 degrees they increase in number to about 
75 degrees, and are occasionally observed even at 
the sun's poles. Faculse and prominences are more 
closely related than the sun spots and prominences. 
There are wide variations in both magnitude and 
type of the prominences. Heights above the sun's 
limb of a few thousand miles are very common, and 
they rarely reach elevations as great as 100,000 
miles, though a very occasional one reaches even 
greater heights. 

Classification of the prominences divides them 
into two broad types, the quiescent and the eruptive. 
The former are for the most part hydrogen, and 
the latter metallic. The quiescent prominences re- 
semble closely the stratus and cirrus type of terres- 
trial clouds, and are frequently of enormous extent 
along the sun's edge. They are relatively long-lived, 
persisting sometimes for days without much change. 
The eruptive prominences are more brilliant, chang- 
ing their form and brightness rapidly. Often they 
appear as brilliant spikes or jets, reaching altitudes 
that average about 25,000 miles. Rarely seen near 
the sun's poles, they are much more numerous nearer 



SUN SPOTS AND PROMINENCES 183 

the sun spots. Speed of motion of their filaments 
sometimes exceeds one hundred miles a second, and 
the changing variety of shapes of the eruptive prom- 
inences is most interesting. Oftentimes they change 
so rapidly that only photography can do them justice. 

Prominence photography began with Young a 
half century ago, who obtained the first successful 
impression on a microscope slide with si sensitized 
film of collodion; as was necessary in the earlier 
wet-plate process of photography, which required 
exposures so long that little progress was effected 
for about twenty years. Then it was taken up by 
Deslandres of Paris and Hale of Chicago independ- 
ently, both of whom succeeded in devising a com- 
plex type of apparatus known as the spectrohelio- 
graph, by which all the prominences surrounding 
the entire limb of the sun can be photographed at 
any time by light of a single wave-length, together 
with the disk of the sun on the same negative. 

The prominences appear to be intimately con- 
nected with a gaseous envelope surrounding the solar 
photosphere, in which sodium and magnesium are 
present as well as hydrogen. The depth of the 
chromosphere is usually between 5,000 and 10,000 
miles, and its existence was first made out during 
the total solar eclipses of 1605 and 1706, when it ap- 
peared as an irregular rose-tinted fringe, though not 
at the time recognized as belonging to the sun. 

The constitution of the sun and its envelopes are 
still under discussion, and no complete theory of the 
sun has yet been advanced which commands the 
widest acceptance. Of the interior of the sun we can 
only surmise that it is composed of gases which, be- 
cause of intense heat and compression, are in a state 
unfamiliar on earth and impossible to reproduce in 



184 ASTRONOMY TO-DAY 

our laboratories. Their consistency may be that of 
melted pitch or tar. 

Surrounding the main body of the sun are a series 
of layers, shells, or atmospheres. Outside of all and 
very irregular in structure, indeed probably not a 
solar atmosphere at all, is the solar corona, parts 
of which behave much as if it were an atmosphere, 
but it appears to be bound up in some way with the 
sun's radiation. It has streamers that vary with the 
sunspot period, but its constitution and function are 
very imperfectly known, because it has never been 
seen or photographed except at rare intervals on 
occasion of total eclipses of the sun. 

Beneath the corona we meet the projecting prom« 
inences, to which parts of the corona are certainly 
related, and beneath them the first true layer or 
atmosphere of the sun known as the chromosphere, 
its average depth being about one-hundredth part of 
the sun's diameter. Beneath the chromosphere is 
the layer of the sun from which emanates the light 
by which we see it, called the photosphere. It ap- 
pears to be composed of filaments due to the con- 
densation of metallic vapors, and it is the outer 
extremities of these filaments which are seen as 
the granular structures everywhere covering the 
disk of the sun. Their light shines through the 
chromosphere and the spots are ruptures in this 
envelope. 

Between photosphere and chromosphere is a very 
thin envelope, probably not over 700 miles in thick- 
ness, called the reversing layer. It is this relatively 
thin shell that is responsible for the absorption 
which produces the dark lines in the spectrum of 
the sun. Under normal conditions the filaments of 
the photosphere are radial, that is vertical on the 



SUN SPOTS AND PROMINENCES 185 

sun; but whenever eruptions take place, as during 
the occurrence of spots, the adjacent filaments are 
violently swept out of their normal vertical lines 
and these displaced columns then form what we view 
as the spot's penumbra. From the outer surface of 
the sun's chromosphere rise in eruptive columns 
vapors of hydrogen and the various metals of which 
the sun is composed. These and the spots would 
naturally occur in periods just as we see them. 

We have said that the sun is composed of a mass 
of highly heated or incandescent vapors or gases, 
whose compression on account of gravity must 
render their physical condition quite different from 
any gaseous forms known on the earth or which we 
can reproduce here. As the result of more than 
half a century of studious observation of the sun 
and mapping of its spectrum in every part, and dili- 
gent comparison with the spectra of all known chem- 
ical elements on the earth, we find that the sun con- 
tains no elements not already found here, but that 
a great preponderance of elements known to earth 
are found in the sun. 

The intensity of their spectral lines is one prom- 
inent indication of the presence of elements in the 
sun, and the number of coincidences of spectral 
lines is another. Iron, nickel, calcium, manganese, 
sodium, cobalt, and carbon are among the elements 
most strongly identified. A few of the rarer ter- 
restrial elements are of doubtful existence in the sun, 
and a very few, as gold, bismuth, antimony, and 
sulphur are not found there, and the existence of 
oxygen in the sun is regarded by some experts as 
doubtful. But if the whole earth were vaporized by 
heat, probably its spectrum would resemble that of 
the sun very closely. 



186 ASTRONOMY TO-DAY 

What are the effects of the sun, and sun spots in 
particular, on our weather? Is the influence of their 
periodicity potent or negligible? If we investigate 
conditions pertaining to terrestrial magnetism, as 
fluctuations of the magnetic needle, and the fre- 
quency of aurorse, there is no occasion for doubt of 
the sun's direct influence, although we are not able 
to say just how that influence becomes potent. If, 
however, we look into questions of temperature, 
barometric pressure, rainfall, cyclones, crops, and 
jconsequent financial conditions, we find fully as much 
evidence against solar influence as for it. The slight 
variations of the sun's light and heat due to the 
presence or absence of sun spots can scarcely be 
sensible, and much longer periods of closer observa- 
tion are necessary before such questions can be 
finally decided. The slighter such influences are, if 
they actually exist, and the more veiled they are by 
other influences more or less powerful, the more dif* 
ficult it is to discover their effects with certainty. 

The importance of solar radiation in the predic- 
tion of terrestrial weather has long been recognized^ 
but until very recently no practical application has 
been made. The Smithsonian Astrophysical Ob- 
servatory at Washington, under the direction of Dr. 
Abbot, has for many years carried on at a number 
of stations a series of determinations of the constant 
of solar radiation by the spectro-bolometric method 
originated by Langley. A new station in Calama, 
Chile, has recently been inaugurated, at which the 
solar constant is worked out each day, and tele- 
graphed to the Argentine weather service, where 
it is employed in forecasting for the day. 

Abbot's new method of solar constant determina- 
tion is based on the fact that atmospheric transpar- 



SUN SPOTS AND PROMINENCES 187 

ency varies oppositely to the variations of bright- 
ness of the sky. Increase of haziness presents more 
reflecting surface to scatter the solar rays indirectly 
to the earth. Of course it presents also additional 
surface to obstruct the direct rays from the sun. By 
measuring the brightness of the sky near the sun, 
it becomes possible to infer the coefficients of at- 
mospheric transmission at all wave lengths. The 
direct observations and the complete deduction of 
the solar constant for the day can all be completed 
within two or three hours. 

Clayton of Buenos Aires has now employed these 
results in the Argentine weather predictions for two 
years, and the introduction of this new element in 
forecasting has brought about a pronounced gain in 
the value of the predictions. Its adoption by the 
weather bureaus of other nations will doubtless 
come in due time, and the new method take a firmly 
established rank in practical meteorology. 

Abbot's observations many years ago first called 
attention to the variability of the solar constant 
through a range of several per cent both from year 
to year, and in irregular short periods of weeks or 
even days. Abbot considers this the more likely 
explanation than that atmospheric changes should 
take place simultaneously all over the earth. The 
sun is but a star, the stars that are irregularly 
variable in light and heat are num.erous, and the 
sun itself appears to be one of these. 

Especially important to the agricultural and vine- 
yard interests of Argentine is the question of pre- 
cipitation, and Clayton finds this very dependent on 
solar radiation. At epochs of practically stationary 
solar intensity, there is little or no precipitation; 
but quite generally he finds that great decrease of 



188 ASTRONOMY TO-DAY 

solar radiation is followed in from three to five days 
by heavy precipitation. Direct temperature effects 
are also traced in Buenos Aires and other South 
American cities, lagging from two to three days 
behind the observed solar fluctuations. 

The station at Calama yields about 250 determina- 
tions of the solar constant each year, and the Mount 
Wilson station about half that number. They are 
the only stations of this character at present in ex- 
istence, and others should be established in widely 
separated and cloudless regions, as Egypt, southern 
California and Australia. Uniformity in the 
methods of observing would be highly desirable, 
and the Smithsonian Institution has perfected the 
details of common control of such stations which it 
is expected may be established at an early day. 



CHAPTER XXVII 

THE INNER PLANETS 

VULCAN 

ABOUT the middle of the last century, Le Verrier, 
. a great French astronomer, having added the 
planet Neptune beyond the outside confines of the 
solar system, sought evidence of a lesser planet trav- 
eling round the sun within the orbit of Mercury. 
For many years close watch was kept on the sun in 
the hope of discovering such a body in the act of 
passing across the disk, or in transit, as it is 
technically termed. Lescarbault, a French phy- 
sician, announced that he had actually seen such a 
planet, Vulcan it was called, passing over the sun 
in 1859. Total eclipses of the sun would afford the 
best opportunity for seeing such a body, and on 
several such occasions astronomers thought they had 
found it. But the signal advantages of photography 
have been applied so often to this search, and always 
unsuccessfully, that the existence of Vulcan, or the 
intramercurian planet, is now regarded as mythical. 

MERCURY 

This planet is an elusive body that very few, even 
astronomers, have ever seen. It is not very bright, 
has a rapid motion and never retreats far from the 
sun, so that it was a puzzle to the ancients who saw 
it, sometimes in the twilight after sunset and again 
in the twilight of dawn. When following the sun 

189 



190 ASTRONOMY TO-DAY 

down in the west, in March or April, Mercury is 
likely to be best seen ; twinkling rather violently and 
nearly as bright as a star of the first magnitude. 

Very little is to be seen on the minute disk of this 
planet^ except that it goes through all the phases of 
the moon — crescent, gibbous, full, gibbous, crescent. 
Whether Mercury turns round on its axis or not, 
cannot be said to be known, because the markings 
that are suspected on its surface are too indefinite 
to permit exact observation. More than likely the 
planet presents always the same side or face to the 
sun, so that it turns round on its axis once, while 
traveling once around the sun in its orbit. Mercury's 
day and year would therefore be equal in length. 
Nor have we much evidence on the question of an 
atmosphere surrounding Mercury; probably it is 
very thin, if indeed there is any at all. When Mer- 
cury comes directly between us and the sun, cross- 
ing in transit, the edge of the planet as projected 
against the sun is very sharply defined, and this 
would indicate an absence of atmosphere on Mercury. 

Transits of Mercury can occur in May and 
November only : there was one on November 7, 1914, 
and there will be one on May 7, 1924. The latter 
will be nearly eight hours in length, which is almost 
the limit. Mercury's distance from the sun averages 
36 million miles, the diameter of the planet is 3,000 
miles, and his orbital speed is 30 miles per second, 
the swiftest of all the planets. No moon of Mercury 
is known to exist, although many times diligently 
searched for, especially during transits of the planet. 

VENUS 

Brightest of all the planets, and the most beauti- 
ful of all is Venus. Its path is next outside the orbit 



THE INNER PLANETS 191 

of Mercury, but within that of the earth, so that it 
partakes of all the phases of the moon. Like Mer- 
cury it sometimes passes exactly between us and 
the sun, a rare phenomenon which is known as a 
transit of Venus. 

Being without telescopes, the ancients knew noth- 
ing about these occurrences, but they were puzzled 
for centuries over the appearance of the planet in 
the west after sunset, when they called it Hesperus, 
and in early dawn in the east when they gave it the 
name Phosphorus. 

Venus is known to be girdled with an atmosphere 
denser than ours, and it seems to be always filled 
with dense clouds. It is the reflection of sunlight 
from this perpetually cloudy exterior which gives 
Venus her singular radiance. So brilliant is she 
that even full daylight is not strong enough to over- 
power her rays ; and she may often be seen glisten- 
ing in the clear blue daytime sky, if one knows pretty 
nearly in what direction to look for her. 

Venus is 67 million miles from the sun, and as 
our own distance is 93 million miles, this planet 
can come within 26 million miles of the earth. It 
is therefore at times our nearest known neighbor in 
space, excepting only the Moon and Eros, one of the 
erratic little planets that travel round the sun be- 
tween Mars and Jupiter. Also possibly a comet 
might come much nearer. 

Astronomers always take advantage of this near- 
ness of Venus to us, if a transit across the sun takes 
place ; because it affords an excellent method of find- 
ing out what the distance of the sun is from the 
Earth. A pair of these transits happens about once 
a century, there were transits in 1874 and 1882, and 
the next pair occur in 2004 and 2012. In actual size, 



192 ASTRONOMY TO-DAY 

Venus is almost as large a planet as our own, being 
7,700 miles in diameter, as compared with 7,920 
for the earth. Her velocity in her orbit is twenty- 
two miles per second, and she travels all the way 
round the sun in seven and one half months or 225 
days. 

Venus from her striking brilliancy always leads 
the novice to expect to see great things on applying 
the telescope. But aside from a brilliant disk, now 
a slender crescent, now half full like the moon at 
quarter, and again gibbous as the moon is between 
quarter and full, the telescope reveals but little. 
There is pretty good evidence that the markings 
thought to have been seen on the planet's surface 
are illusory, and so it is wholly uncertain in what 
direction the planet's axis lies; also there is great 
uncertainty about the length of the day on Venus, 
or the period of turning round on its axis. Probably 
it is the same in length as the planet's year. 

Once when Venus passed very close to the sun, 
just barely escaping a transit, Lyman of Yale Uni- 
versity caught sight of it by hiding the sun behind 
a tall building or church spire. The dark side of 
Venus was turned toward us and he could not of 
course see that. But the planet was clearly there, 
completely encircled by a narrow delicate luminous 
ring, which was due to sunlight shining through the 
atmosphere that surrounds the planet. Similar 
ring effects were seen by observers of the transits 
of Venus in 1874 and 1882 ; and from all their ob- 
servations it is concluded that Venus has an at- 
mosphere probably at least twice as dense and ex- 
tensive as that which encircles the earth. Spurious 
satellites of Venus are many, but no real moon is 
known to attend this planet. 




The Surface of the Moon in the Reoicn of Coiernicus. Photograp'.i 

made with the Hooker 100-inch reflecting telescope. {Photo, Mt. Wilson 

Sola)- Observatory.) 














J\ ' 




..^^^^^^ 



A View of the South Central Portion of the Moon at Last 
QuARTEK. {Photo, Mt. Wilson Solar Observatory.) 



CHAPTER XXVIII 

THE MOON AND HER SURFACE 

AS the sun has always reigned as king of day, so 
. is the moon queen of night. Observation of her 
phases, now waxing, now waning, with her stately 
motion always eastward among the stars, began 
with the earliest ages. Often when near the full 
she must have been seen herself eclipsed, and much 
more rarely the occurrence of total eclipses of the 
sun are certain to have suggested the moon's inter- 
vention between earth and sun, shutting off the sun- 
light completely, because these eclipses never took 
place except when the moon was in the same part 
of the sky with the sun. 

If we watch the nightly march of the moon, we 
shall find that she travels over her own breadth in 
about an hour's time. By using a telescope on the 
stars just eastward or to the left of her, she will now 
and then be seen to pass between us and a star — on 
very rare occasions a planet — extinguishing its light 
with great suddenness, the most nearly instantane- 
ous of all phenomena in nature. Draw a line con- 
necting the cusps, or horns of the lunar crescent, and 
then a line eastward at right angles to this, and it 
will show the direction of the moon's own motion in 
its orbit round the earth quite accurately. 

As the phase advances, note the inside edge of the 
advancing crescent: this will be quite rough and 
jagged, compared to the outside edge which is the 

193 Sci. Vol. 2—7 



194 ASTRONOMY TO-DAY 

moon's real contour and relatively very smooth. The 
position of the inside curve will change from night 
to night, and it marks the line of sunrise on the 
moon during the fortnight elapsing between new 
moon and full ; while from full through last quarter 
and back to new moon, this advancing line marks 
the region of sunset on the moon. The general shape 
of this line is never a circle but always elliptical, 
and astronomers call it the terminator. All along 
the terminator, sunlight strikes the lunar surface 
at a small angle, whether near sunrise or sunset; 
so that owing to the mountains and other high 
masses of the moon's surface, the terminator 
is always a more or less jagged and irregular 
line. 

Onward from new moon toward full the horns of 
the crescent are always turned upward or eastward. 
When the general line of the terminator becomes 
a straight line from cusp to cusp, the moon is said 
to have reached first quarter or quadrature. On- 
ward toward full the terminator will be seen to bend 
the other way, and in about a week's time it will have 
merged itself with the m.oon's limb. The moon is 
then said to be full. Afterward the phase phenom- 
ena recur in the reverse order, with third quarter 
midway between full and new moon again ; the phase 
of the moon being called gibbous all the way from 
first quarter to third quarter, except when exactly 
full. 

As we know that the moon is, like the earth, a 
nonluminous body, and shines only by virtue of the 
sunlight falling upon it, clearly an entire half of the 
moon's globe must be perpetually illumined by sun- 
light. The varying phases then are due simply to 
that part of the illuminated hemisphere which is 



THE MOON AND HER SURFACE 195 

turned toward us. New moon is entirely invisible 
because the sunward hemisphere is turned wholly 
away from us, while at full moon we see the lunar 
disk complete because we are on the same side of 
the moon that the sun is and practically in line with 
both sun and moon. 

If we could visit the moon, we should see the earth 
in exactly complementary phase. At new moon here 
we should be enjoying full earth there, and full moon 
here would be coincident with new or dark earth 
there. The narrow crescent of new moon here would 
be the period of gibbous earth there; and it is the 
reflection of sunlight from this gibbous earth which 
illuminates the part of the moon but faintly seen 
at this time, popularly known as the "old moon in 
the new moon's arms.'' Its greater visibility at some 
times than at others is due to greater prevalence of 
clouded area in the reflecting regions of the earth 
turned toward the moon, and the higher reflective 
power of clouds than that possessed by mere land 
and water. 

As the moon goes all the way round the sky every 
month, the same as the sun does in a year, and 
travels in nearly the same path, clearly it must also 
go north and south every month as the sun does. 
So in midsummer when the sun runs high upon the 
meridian, we expect to find full moons running low, 
and likewise in midwinter the full moon always runs 
high, as almost everyone has sometimes or other 
noticed. 

This eastward or true orbital motion of the moon 
is responsible for another relation which soon comes 
to light when we begin to observe the moon; and 
that is the later hour of rising or setting each night. 
Our clock time is regulated by the sun, which also 



196 ASTRONOMY TO-DAY 

is moving eastward about 1° daily, or twice its own 
breadth. So the moon's eastward gain on the sun 
amounts to about 12 degrees daily, and one degree 
being equal to 4 minutes, the retarded time of moon- 
rise or moonset each day amounts to very nearly 50 
minutes on the average; though sometimes the de- 
lay will be less than a half hour and at other times 
it will exceed an hour and a quarter. The season 
of least retardation of rising of the full moon is in 
the autumn, and so the moon that falls in late Sep- 
tember or October is known as the Harvest moon, 
and the next succeeding full moon is called the 
Hunter's moon. 

Lunation is a term sometimes given to the moon's 
period from any definite phase round to the same 
phase again. Its length is the true period of the 
moon's revolution once around the earth, from the 
sun all the way round till it overtakes the sun again. 
The synodic period is another name for lunation, 
and its true length is 29 and one-half days, or very 
accurately 29 d. 12 h. 44 m. 2.7 s. as calculated by 
astronomers with great exactness from many thou- 
sand revolutions of the moon. But if we want the 
true period of the moon round the earth as referred 
to a star, it is much shorter than this, amounting to 
only 27 days and nearly one-third. This is called 
the moon's sidereal period of revolution, because it 
is the time elapsed while she is traveling eastward 
from a given star around to coincidence with the 
same star again. 

If we study the moon's path in the sky more 
critically, we shall find that it does not quite follow 
the ecliptic, or the sun's path, but that twice each 
month she deviates from the ecliptic, once to the 
north and once to the south of it, by roughly ten 



THE MOON AND HER SURFACE 197 

times her own breadth. More accurately this angle 
is 5^8'40", an almost invariable quantity, and it is 
therefore known as an astronomical constant, or the 
inclination of the moon's orbit to the ecliptic. So 
the mxoon's orbit must intersect the ecliptic, and 
as both are great circles in the sky, the points of in- 
tersection are known as the moon's nodes, one as- 
cending and the other descending, and the nodes are 
180 degrees apart. 

The figure of the moon's orbit is not circular, al- 
though it deviates only slightly from that form. But 
like the paths of all other satellites round their 
primary planets, and of the planets themselves round 
the sun, the moon's orbit is also an ellipse. The dis- 
tance of the moon's center from the earth's center 
is therefore perpetually changing; the point of 
nearest approach is called perigee, and that of farth- 
est recession, apogee. 

The moon's distance from the earth is easier 
and simpler to be ascertained than that of any other 
heavenly body, because it is the nearest. An out- 
line of the method of finding this distance is not 
difficult to present ; and it resembles in every partic- 
ular the method a surveyor uses to find the distance 
of some inaccessible point which he cannot measure 
directly. Up and down a stream, for example, he 
measures the length of a line, and from each end of 
it he measures the angle between the other end of 
the line and the object on the opposite side of the 
stream whose distance he wishes to find out. Then 
he applies the science of trigonometry to these three 
measures, two of angles and one the length of the 
side or base included between them, and a few min- 
utes' calculation gives the distance of the inacces- 
sible object from either end of the base line. 



198 ASTRONOMY TO-DAY 

Now in like manner, to transfer the process to the 
sky, let the two ends of the base be represented by- 
two astronomical observatories, for example, Green- 
wich in the northern hemisphere and Cape Town in 
the southern. The base line is the chord or straight 
line through the earth connecting the two observa^ 
tories, and we know the length of this line pretty ac- 
curately, because we know the size of the earth. The 
angles measured are somewhat different from those 
in the terrestrial example, but the process amounts 
to the same thing because the astronomers at the 
two observatories measure the angular distance of 
the center of the moon from the zenith, each using 
his own zenith at the same time; and the same 
science of trigonometry enables them to figure out 
the length of any side of the triangles involved. The 
side which belongs to both triangles is the distance 
from the center of the earth to the center of the 
moon, and the average of many hundred measures 
of this gives 238,800 miles, or about ten times the 
distance round the equator of the earth. 

We have said that the orbit in which the moon 
travels round the earth is practically a circle, but 
the earth's center is found not at the center of 
this orbit, but set to one side, or eccentrically, so 
that the distance spanning the centers of the two 
bodies is sometimes as small as 221,610 miles at 
perigee, and 252,970 miles at apogee. The moon's 
speed in this orbit averages rather more than half 
a mile every second of time — more accurately 3,350 
feet a second, or 2,290 miles per hour. 

Once the moon's distance is known, its size or 
diameter is easy to ascertain. An angular measure 
is necessary, of course, that of the angle which 
the disk of the moon fills as seen from the earth. 



THE MOON AND HER SURFACE 199 

There are many types of astronomical instruments 
with which this angle can be measured, and its 
vakie is something more than half a degree (31' 7"). 
The moon's actual diameter figures out from this 
2,163 miles; and it would therefore require nearly 
fifty moons merged in one to make a ball the size 
of the earth. 

Still, no other planet has a satellite as large in 
proportion to its primary as the moon is in rela- 
tion to the earth. But the materials that com- 
pose the moon have less than two-thirds the 
average density of those that make up the earth, 
so that eighty-one moons fused together would be 
necessary to equal the mass or weight of the earth. 
If we figure out the force of attraction of the moon 
for bodies on its surface, we find it equals about 
one-sixth that of the earth. Athletes could per- 
form som.e astounding feats there — miracles of high 
jump and hammer-throw. 

Our interest in the moon's physical characteristics 
never wanes. Her nearness to us has always fas- 
cinated astronomer and layman alike. Early users 
of the telescope were readily led into error regard- 
ing the general characteristics of the lunar surface ; 
and it is easy to see why they thought the smooth 
level planes must be seas, and gave them names to 
that effect v/hich persist to-day, as Mare Crisium, 
Mare Serenitatis and so on. "We may be sure that 
no water exists on the moon's surface, although 
some astronomers think that solid water, as ice or 
snow, may still exist there at a temperature too low 
for appreciable evaporation. 

Perhaps water, seas, and oceans were once there, 
but their secular dissemination and loss as vapor 
have gone on through the millions of millions of 



200 ASTRONOMY TO-DAY 

years till even the moon's atmosphere appears to 
have vanished completely. At least there is much 
better evidence of absence of atmosphere on the 
moon than of its presence — not enough at any rate 
to equal a thousandth part of the barometric pres- 
sure that we have at the earth's surface. Frequent 
observations of stars passing behind the moon 
in occultation have satisfied astronomers on this 
point. 

We often say of the brilliant full moon, it is as 
bright as day. The photometer or instrument for 
accurate comparison of lights, their amount and in- 
tensity, tells a different story. Indeed, if the en- 
tire dome of the sky were filled with full moons, we 
should be receiving only one-eighth of the light the 
sun gives us, and it would require more than 600,- 
000 average full moons to equal the light radiation 
of the sun. Heat from the moon, however, is quite 
different. Early attempts to measure it detected 
none at all, but with modern instruments there is 
little trouble in detecting heat from the moon, 
though measurement of it is not easy. 

Much of the moon's heat is sun heat, directly re- 
flected from the moon, as sunlight is, but most of it 
is due to radiation of solar heat previously absorbed 
by the materials of the lunar surface. The actual 
temperature of the moon's surface suffers great 
variation. A fortnight's perpetual shining of the 
sun upon the lunar rocks would certainly heat them 
above the temperature of boiling water, if the moon 
had an atmosphere to conserve and store this heat ; 
but the entire absence of such an air blanket proba- 
bly permits the sun's heat to be radiated away nearly 
as fast as it is received, leaving the temperature at 
the surface always very low. 



THE MOON AND HER SURFACE 201 

What physical influences the moon really has upon 
the earth must be very slight, barring the tides. 
But there is little hope of getting people generally 
to take that view, because the moon appears to be 
the planet of the people, and opinion that the moon 
controls the weather, for instance, amounts with 
them to practical certainty. More than likely all 
these notions are but legitimate survivals of super- 
stition and astrology. In addition to the tides, our 
magnetic observatories reveal slight disturbances 
with the swinging of the moon from apogee to peri- 
gee and back; but long series of weather observa- 
tions have been faithfully interrogated, with nega- 
tive or contradictory results. If one believes that 
the moon's changes affect the weather, it is easy to 
remember coincidences, and pass over the many 
times when no change has taken place. The moon 
changes pretty frequently anyhow. As Young well 
puts it: "A change of the moon necessarily occurs 

about once a week All changes, of the weather 

for instance, must therefore occur within three and 
three-fourth days of a change of the moon, and fifty 
per cent of them ought to occur within forty-six 
hours of a change, even if there were no causal con- 
nection whatever." 

When we turn to the strongly diversified surface 
of the moon itself, we find much to rivet the at- 
tention, even with slender optical aid. Everyone 
wants to know how near the telescope, the biggest 
possible telescope, brings the moon to us. That will 
depend on many things, first of all on the magnify- 
ing power of the eyepiece employed on the telescope, 
and eyepieces are changed on telescopes just as they 
are on microscopes, though not for the same reasons. 
The theoretical limit of the power of a telescope is 



202 ASTROKOMY TO-DAY 

usually considered as 100 for each inch of diameter 
or aperture of the object glass. 

A 40-inch telescope, as that of the Yerkes 
Observatory, the largest refracting telescope in 
existence, should bear a magnifying power not to 
exceed 4,000. But this limit is practically never 
reached, one-half of it or fifty to the inch of aperture 
being a good v^orking limit of power, even under 
exceptional conditions of steadiness of atmosphere. 
If we reduce the effective distance of the moon from 
240,000 miles to 100 miles, that is about the utmost 
that can be expected. But even at that distance we 
can make out only landscape details, nothing what- 
ever like buildings or the works of intelligence. 

The larger relations of light and shade, so 
obvious to the naked eye on the moon, vanish on 
looking at it with the telescope, but we are at once 
captivated by the novel character of the surface 
and the seemingly great variety of detail that is 
clearly visible. As soon as the ne^v moon comes 
out in the west, one may begin to gaze with in- 
terest and watch the terminator or sunrise line 
gradually steal over the roughened surface, bring- 
ing new and striking craters into view each night. 
Around the time of quarter moon, or a little past 
it, is one of the best times for telescopic views of 
the moon, because the huge craters, Tycho and 
Copernicus, are then in fine illumination. Close 
to the phase of full moon is never a good time, 
because there are no shadows of the rough surface 
then, and its entire structure seems to be quite 
flat and uninteresting, except for the streaks or 
rills which radiate from Tycho in every direction, 
and are the only lunar features that are best seen 
near full. 



THE MOON AND HER SURFACE 203 

In a broad, general way, the moon's surface, if 
compared with the earth's, differs in having no 
water. Our extensive oceans are replaced there by 
smooth, level plains which were at first thought to 
be seas and so named. There are ten or twelve of 
them in all. Then we find mountain ranges, so 
numerous on the earth, relatively few on the moon. 
Those that exist are named, in part, for terrestrial 
mountain ranges, as the Alps, Caucasus, and the 
Apennines. 

But the nearlj^ circular crater, a relatively rare 
formation on the earth, is seen dotted all over the 
moon in every size, from a fraction of a mile in 
diameter up to sixty, seventy, and in extreme cases 
a hundred miles. No mere description of plains 
and mountains and craters affords an adequate 
idea of the moon's surface as it actually is; a 
telescopic view is necessary, or some of the modern 
photographs which give an even better notion of 
the moon than any telescopic view. Many of the 
lunar craters are without doubt volcanic in origin, 
others seem to be ruins of molten lakes. Many 
thousands of the smaller ones appear as if formed 
by a violent pelting of the surface when semi- 
plastic, perhaps by enormous showers of meteoric 
matter. More than 30,000 craters cover the half 
of the lunar surface visible from the earth, and 
hundreds of them are named for philosophers and 
astronomers. 

Measurement of the height of lunar mountains 
has been made in numerous instances, especially 
when their shadows fall on plains or surfaces that 
are nearly level, so that the length of the shadow 
can be measured. In general, the height of lunar 
peaks is greater than that of terrestrial peaks, 



204 ASTRONOMY TO-DAY 

owing probably to the lesser surface gravity on 
the moon. About forty lunar peaks are higher 
than Mont Blanc. 

Most astronomers regard it as certain that no 
changes ever take place on the moon; probably no 
very conspicuous changes ever do. Some, however, 
have made out a fair case for comparatively recent 
changes in surface detail. Extreme caution is neces- 
sary in drawing conclusions, because the varying 
changes of illumination from one phase to another 
are themselves sufficient to cause the appearance 
of change. At intervals of a double lunation, equal 
to fifty-nine days, one and one-half hours, the 
terminator goes very nearly through the same 
objects, so that the circumstances of illumination 
are comparable. In Mare Serenitatis the little 
crater named Linne was announced to have disap- 
peared about a half century ago; subsequently it 
became visible again and other minor changes were 
reported, perhaps due to falling in of the walls of 
the crater. 

If one were to visit the moon, he must needs 
take air and water along with him, as well as 
other sustenance. No atmosphere means no dif- 
fused light; we could see nothing unless the sun's 
direct rays were shining upon it. Anyone stepping 
into the shadow of a lunar crag would become 
wholly invisible. No sound, however loud, could be 
heard; sound in fact would become impossible. A 
rock might roll down the wall of a lunar crater, 
but there would be no noise; though we should 
know what had happened by the tremor produced. 
So slight is gravity there that a good ball player 
might bat a baseball half a mile or more. Looking 
upward, all the stars would be appreciably brighter 



THE MOON AND HER SURFACE 205 

than here, and visible perpetually in the daytime 
as well as at night. 

If one were to go to the opposite side of the 
moon, he would lose sight of the earth until he 
came back to the side which is always turned to- 
ward the earth. Even then the earth would never 
rise and set at any given place, as the moon does 
to us, but would remain all the time at about the 
same height above the lunar horizon. The earth 
would go through all the phases that the moon 
shows to us here, full earth occurring there when 
it is new moon here. Our globe would appear to 
be nearly four times broader than the moon seems 
to us. Its white polar caps of ice and snow, its 
dark oceans, and the vast cloud areas would be 
very conspicuous. Faint stars, the zodiacal light, 
and the filmxy solar corona would be visible, prob- 
ably even close up to the sun's edge; but although 
his rays might shine upon the lunar rocks without 
intermission for a fortnight, probably they would 
still be too cold to touch with safety. On the side 
of the moon turned away from the sun, the tem- 
perature of the moon's surface would fall to that 
of space, or many hundred degrees below zero. 



CHAPTER XXIX 

ECLIPSES OF THE MOON 

OF all the weird happenings of the nighttime sky, 
eclipses of the moon are the most impressive. 
Rarely is there a year without one. What is the 
cause ? Simply the earth getting in between sun and 
moon, and thereby shutting off the sunlight which at 
all other tim.es enables us to see the moon. As the 
earth is a dark body it must cast a black shadow on 
the side away from, the sun, and it is the moon's pass- 
ing into this shadow or som.e part of it that causes a 
lunar eclipse. 

Sun and earth being so different in size, the earth's 
shadow must stretch away from it into space, grow- 
ing sm.aller and smaller, until at length it comes to 
an end — the apex of a cone 857,000 miles long. 
If we cut off this shadow at the moon's distance from 
the earth, we find it about 6,000 miles in diameter 
at that point; and this accounts for the fact that the 
curvature on the side of the moon, when the eclipse 
is coming on and where it is dropping into the 
shadow, is always much less rapid than the curva- 
ture of the moon's own disk is. 

When an eclipse is approaching, the eastern limb 
will be duskily darkened for half an hour or more, 
because the moon must first pass through the outer 
penumbra, or half-shadow which everywhere sur- 
rounds the true shadow itself. If the moon hits 
only the upper or lower part of the shadow, the 

206 



ECLIPSES OP THE MOON 207 

eclipse will be only partial, and during the progress 
of the eclipse it will seem as if the uneclipsed part 
had swung or twisted around in the sky, from the 
western limb of the moon to the eastern. But when 
the moon passes through the middle regions of the 
shadow, the eclipse is always total, and direct sun- 
light is wholly cut off from every part of the moon's 
face, for a greater or less length of time, according 
to the part of the shadow through which it passes. 
When passing centrally through the shadow, the 
total eclipse will last about two hours, as the moon's 
diameter is about one-third of the breadth of the 
shadow; and the eclipse will be partial about two 
hours longer, an hour at beginning and an hour at 
the end, because the moon moves over her own 
breadth in about an hour. 

While the moon is wholly immersed in the shadow, 
her body is nevertheless visible, as a dull tarnished 
copper disk; and this is caused by the reddish sun- 
light which grazes the earth all around and is re- 
fracted or bent by our atmosphere into the shadow 
itself. If this belt or ring of terrestrial atmosphere 
happens to be everywhere filled with dense clouds, 
as was the case in 1886, even the familiar copper 
moon of a total lunar eclipse disappears completely 
in the black sky. 

Quite different from a solar eclipse, all the phases 
of a lunar eclipse are visible at the same time on the 
earth wherever the moon is above the horizon. 
Eclipses of the moon are therefore seen with great 
frequency at any given place as compared with solar 
eclipses, which are restricted to relatively narrow 
areas of the earth's surface. Nor are lunar eclipses 
of very much significance to the astronomer, mainly 
because of the slowness and indefiniteness of the 



208 ASTRONOMY TO-DAY 

phenomena. It is a good time to observe occulta- 
tions of faint stars at the moon's edge or limb, 
and several such programs have been carried out 
by cooperation of observatories in widely separate 
regions of the world: the object being improvement 
in our knowledge of the distance of the moon, and 
in the accuracy of the mathematical tables of her 
motion. Search by photography for a possible satel- 
lite, or moon of the moon, has been made on several 
occasions, though without success. 

A lunar eclipse was first observed and photo- 
graphed from an aeroplane, May 2, 1920. At the 
request of the writer, two aviators of the United 
States navy ascended to a height of 15,000 feet above 
Rockaway, and secured many advantages accruing 
from great elevation in viewing a celestial phenom- 
enon of this character. 



CHAPTER XXX 

TOTAL ECLIPSES OF THE SUN 

PRIMITIVE peoples indulged in every variety of 
explanation of mysterious happenings in the sky. 
To the Chinese and all through India, a total eclipse 
of the sun is caused by "a certain dragon v^ith very 
black claws/* who, except for their frightening him 
away by every conceivable sort of hideous noise, 
would most certainly **eat up the sun/' The eclipse 
always goes off, the sun has never been eaten yet. 
Can you convince a Chinaman that Rahu, the 
Dragon, wouldn't have eaten up the sun, if his un- 
earthly din hadn't frightened him away? 

In Japan the eclipse drops poison from the sky 
into wells, so the Japanese cover them up. Fon- 
tenelle relates that in the middle of the seventeenth 
century a multitude of people shut themselves up in 
cellars in Paris during a total eclipse. 

In the Shu-king, an ancient Chinese work, occurs 
the earliest record of a total eclipse of the sun, in 
the year B. C. 2158. The Nineveh ecHpse of B. C. 763 
is perhaps the first of the ancient eclipses of 
which we possess a really clear description on the 
Assyrian eponym tablets in the British Museum. 
It is the eclipse possibly referred to in the Book 
of Amos, viii. 

But of all the ancient eclipses none perhaps ex- 
ceeds in interest the famous eclipse of Thales, B. C. 
585, May 28. It is the first eclipse to have been 
predicted, probably by means of the saros, or 18- 

209 



210 ASTRONOMY TO-DAY 

year period of eclipses, which is useful as an 
approximate method even at the present day. But 
the accident of a war between the Lydians and the 
Medes has added greatly to the historic interest, 
because the combatants were so terrified by the 
sudden turning of day into night that they at once 
concluded a peace cemented by two marriages. 

Very many of the ancient eclipses have been of great 
use to the historian in verifying dates, and mathe- 
matical astronomers have employed them in correct- 
ing the lunar tables, or intricate mathematical data 
by which the motion of the moon is predicted. 

Coming down to the middle of the sixth cen- 
tury, we find the first eclipse recorded in England, 
in the "Saxon Chronicle,'* A. D. 538. During the 
epoch of the Arabian Nights several eclipses were 
witnessed at Bagdad, A. D. 829 to 928, and many a 
century later by Ibu-Jounis, court astronomer of 
Hakem, the Caliph of Egypt. Nothing is more in- 
teresting than to search the quaint records of these 
ancient eclipses. One occurring in 1560, when 
Tycho Brahe was but fourteen, had much to do 
with turning his permanent interest toward mathe- 
matics and astronomy. The eclipse of 1612 was the 
first "seen through a tube," the telescope having 
been invented only a few years before. "Paradise 
Lost" was completed about 1665, and the censor- 
ship was still in existence; and it is matter of 
record that the oft-quoted passage, 

"As when the Sun, new risen, 
Looks through the horizontal misty air, 
Shorn of his beams ; or from behind the Moon, 
In dim eclipse, disastrous twilight sheds 
On half the nations, and with fear of change 
Perplexes monarchs." 

P. L., i. 694 



TOTAL ECLIPSES OF THE SUN 211 

was strongly urged as sufficient reason for sup- 
pressing the entire epic. 

London was favored with the outflashing corona, 
May 8, 1715, and a pamphlet was issued in pre- 
diction, entitled "The Black Day, or a Prospect of 
Doomsday." 

The first American eclipse expedition was on 
occasion of the totality of Oct. 27, 1780, sent out 
by Harvard College and the American Academy of 
Arts and Sciences under Professor Samuel Wil- 
liams to Penobscot. There was a fine total eclipse 
from Albany to Boston on June 16, 1806, and many 
important observations of it were made in this 
country. 

But it was not till the European eclipse of 1842 
that research got fully under way, because the germ 
of the new astronomy, particularly as applied to 
the sun, had begun its development ; and the signifi- 
cance of the corona was obvious, if it could be 
proved a true appendage of the sun. Photography 
had not long been discovered, and the corona of 
1851 was the first to be automatically registered 
on a daguerreotype. In 1860 it was proved that 
prominences and corona both belong to the sun and 
not to the moon. 

The great Indian eclipse of 1868 brought the im- 
portant discovery that the prominences can be 
observed at any time without an eclipse by means 
of the spectroscope. In 1869 bright lines were 
found in the spectrum of the corona, one line in 
the green indicating the presence of an element not 
then known on the earth and hence called coro- 
nium. In 1870 the reversing layer or stratum of 
the sun was discovered. In 1878 a vast ecliptic 
extension of the streams of the corona many mil- 



212 ASTRONOMY TO-DAY 

lions of miles both east and west of the sun was 
first seen. This is now known to be the type of 
corona characteristic of minimum spots on the sun. 
In 1882 the spectrum of the corona was first photo- 
graphed and in 1889 excellent detail photographs 
of the corona were taken. In 1893 it was shown 
that the corona quite certainly rotates bodily with 
the sun. In 1896 actual spectrum photographs of 
the reversing layer established its existence beyond 
doubt — "flash spectrum" it is often called. In 1898 
the long ecliptic streamers of the corona were suc- 
cessfully photographed for the first time. In 1900 
the depth of the reversing layer was found to aver- 
age 500 miles, the heat of the corona was first 
measured by the bolometer, and many observations 
showed that the coronal streamers, in part at least, 
partake the nature of electric discharges. 

All subsequent total eclipses have been carefully 
observed, in whatever part of the world they may 
happen, and each has added new results of signifi- 
cance to our theories of the corona and its rela- 
tion to the radiant energy of the sun. In very 
recent eclipses the cinematograph has been brought 
into action as an efficient adjunct of observation; 
in 1914 the first successful "movie" of the eclipse 
was secured in Sweden, and in 1918 Frost of the 
Yerkes Observatory first applied the cinematograph 
to registry of the "flash spectrum," and Stebbins 
tested out his photo-electric cell on the corona, mak- 
ing the brightness 0.5 that of the full moon. In 
1914 (Russia) and again in 1919 (on the Atlantic) 
the obvious advantages of the aeroplane in ecliptic 
observation and photography were sought by the 
writer, though unsuccessfully. The photographic 
tests, however, conducted in preparation for these 



TOTAL ECLIPSES OF THE SUN 213 

expeditions proved the entire practicability of 
securing eclipse results of much value, indepen- 
dently of clouds below. 

Eclipses in the near future will be total in Aus- 
tralia about six minutes on September 21, 1922 ; in 
California and Mexico about four minutes on Sep- 
tember 10, 1923, and along a line from Toronto to 
Nantucket about two minutes on the morning of 
January 24, 1925. 

To all spectators, savage or civilized, scientist 
or layman, a total eclipse is wonderful and im- 
pressive. Langley said: "The spectacle is one of 
which, though the man of science may prosaically 
state the facts, perhaps only the poet could render 
the impression." Very gradually the moon steals 
its way across the face of the sun, the lessened 
light is hardly noticed. If one is near a tree through 
whose foliage the sunlight filters, an extraordinary 
sight is seen; the ground all about is covered with 
luminous crescents, instead of the overlapping 
disks which were there before the eclipse came on ; 
in both cases they are images of the disk of the 
sun at the time, and the narrowing crescents will 
be watched with interest as totality approaches. 
Then the shadow bands may be seen flitting across 
the landscape, like "visible wind." They are prob- 
ably related to our atmosphere and the very slen- 
der crescent from which true sunlight still comes. 

Then for a few seconds the moon's actual 
shadow may be caught in its approach, very sud- 
denly the darkness steals over the landscape and 
— ^totality is on. How lucky if there are no clouds ! 
Every eye is riveted on "the incomparable corona, 
a silvery, soft, unearthly light, with radiant 
streamers, stretching at times millions of uncom- 



214 ASTRONOMY TO-DAY 

prehended miles into space, while the rosy flam- 
ing protuberances skirt the black rim of the moon 
in ethereal splendor." 

Then it is now or never with observer and photog- 
rapher. Months of diligent preparations at home fol- 
lowed by weeks of tedious journey abroad, with days 
of strenuous preparation and rehearsals at the sta- 
tion — all go for naught unless the whole is tuned up 
to perfect operation the instant totality begins. It 
may last but a minute, or even less; in 1937, how- 
ever, total eclipse will last 7 minutes 20 seconds, the, 
longest ever observed, and within half a minute of 
the longest possible. All is over as suddenly as it 
came on. The first thing is to complete records, de- 
velop plates, and see if everything worked perfectly. 

There is great utility back of all eclipse research, 
on account of its wide bearing on meteorology and 
terrestrial physics, and possibly the direct use of 
solar energy for industrial purposes. With this 
purpose in view the astronomer devotes himself un- 
sparingly to the acquisition of every possible fact 
about the sun and his corona. 

Considering the earth as a whole, the number of 
total eclipses will average nearly seventy to the 
century. But at any given place, one may count him- 
self very fortunate if he sees a single total eclipse, 
although he may see several partial ones without 
going from home. Then, too, there are annular or 
ring eclipses, averaging seven in eight years. But 
had one been born in Boston or New York in the 
latter part of the eighteenth century, he might have 
lived through the entire nineteenth century and a 
long way into the twentieth without seeing more 
than one total eclipse of the sun. In London in 1715 
no total eclipse had been visible for six centuries. 



TOTAL ECLIPSES OF THE SUN 215 

However, taking general averages, and recalling the 
comparatively narrov^ belt of total eclipse, every 
part of the earth is likely to come within range 
of the moon's shadow once in about three and a 
half centuries. 

The longest total eclipses always occur near the 
equator; this is because an observer on the equator 
is carried eastward by the earth's rotation at a 
velocity of about 1,000 miles per hour, so that he 
remains longer in the moon's shadow which is pass- 
ing over him in the same direction with a velocity 
about twice as great. 

The general circumstances of total eclipses are 
readily foretold by means of the ancient Chaldean 
period of eclipses known as the saros. It is 18 
years and 10 or 11 days in length (according to 
the number of leap years intervening). In one 
complete saros, forty-one solar eclipses will gen- 
erally happen, but only about one-fourth of them 
will be total. The saros is a period at the end of 
which the centers of sun and moon return very 
nearly to their relative positions at the beginning 
of the cycle. So, in general, the eclipse of any 
year will be a repetition of one which took place 18 
years before, and another very similar in circum- 
stances will happen 18 years in the future. Three 
periods of the saros, or 54 years and 1 month, will 
usually bring about a return of any given eclipse 
to any t)articular part of the earth, so far as longi- 
tude is concerned, though the returning track will 
lie about 600 miles to the north or south of the one 
54 years earlier. 

Paths of total eclipses frequently intersect, if 
large areas like an entire country are considered; 
Spain, for instance, where total eclipses have oc- 



216 ASTRONOMY TO-DAY 

curred in 1842, 1860, 1870, 1900 and 1905. Be- 
sides crossing Spain, the tracks of totality on May 
28, 1900, and August 30, 1905, were unique in inter- 
secting exactly over a large city — Tripoli in Bar- 
bary, on both of which occasions the writer's ex- 
peditions to that city were rewarded with perfect 
observing conditions in that now Italian province 
on the edge of the great desert. 

Kepler was the first astronomer to calculate 
eclipses with some approach to scientific form, as 
exemplified in his Rudolphine Tables. His method 
was of course geometrical. But La Grange, who 
applied the methods of more refined analysis to the 
problem, was the first to develop a method by which 
an eclipse and all its circumstances could be accu- 
rately predicted for any part of the earth. To many 
minds, the prediction of an eclipse affords the best 
illustration of the superior knowledge of the astron- 
omer: it seems little short of the marvelous. But 
recalling that the motion of the moon follows the 
law of gravitation, and that its position in the sky 
is predictable for years in advance with a high de- 
gree of precision, it will readily be seen how the 
arrival of the moon's shadow, and hence the total 
eclipses of the sun, can be foretold for any place 
over which the shadow passes. 

All these data derived by the mathematician are 
known as the elements of the eclipse, and they 
are prepared many years in advance and published 
in the nautical almanacs and astronomical ephe- 
merides issued by the leading nations. Buchanan's 
"Treatise on Eclipses" will supply all the technical 
information regarding the prediction of eclipses 
that anyone desirous of inquiring into this phase 
of the problem may desire. 



TOTAL ECLIPSES OF THE SUN 217 

So important are total eclipses in the scheme of 
modern solar research, and so necessary are clear 
skies in order that expeditions may be favored with 
success, that every effort is now made to ascertain 
the weather chances at particular stations along 
the line of eclipse many years in advance. This 
method of securing preliminary cloud observations 
for a series of years has proved especially useful 
for the eclipses of 1893, 1896, 1900, and 1918 ; and 
had it been employed in Russia for totality of 
1914, many well-equipped expeditions might have 
been spared disaster. The California and Mexico 
totality of 1923 does not require this forethought, 
as the regions visited are quite likely to be free 
from cloud ; but observations are now in process of 
accumulation for the total eclipse of 1925. The out- 
look for clear skies on that occasion, the total 
eclipse nearest New York for more than a century, 
is not very promising. The path of totality passes 
over Marquette, Michigan, Rochester and Pough- 
keepsie, New York, Nev/port, Rhode Island, and 
Nantucket about nine in the morning. 

Everyone who saw it will remember the last 
total eclipse in this part of the world — on June 8, 
1918, visible from Oregon to Florida. Many will 
recall the last total eclipse that was visible before 
that in the eastern part of the United States, on 
May 28, 1900, visible in a narrow path from New 
Orleans to Norfolk. One's father or grandfather 
will perhaps remember the total eclipse of July 29, 
1878, which passed over the United States from 
Pike's Peak to Texas (it was the writer's maiden 
eclipse), and another on August 7, 1869, which 
passed southeasterly over Iowa and Kentucky. On 
all these occasions the paths of total eclipse were 



218 ASTRONOMY TO-DAY 

dotted with numerous observing parties, many of 
them equipped with elaborate apparatus for study- 
ing and photographing the solar corona and prom- 
inences, together with a multitude of other phe- 
nomena which are seen only when total eclipses 
take place. 

Looking forward rather than backward, a strik- 
ing series, or family, of eclipses happens in the 
future: it is the series of May, 1901 and 1919, re- 
curring again on June 8, 1937 (over the Pacific 
Ocean), June 20, 1955 (through India, Siam., and 
Luzon), and June 30, 1973 (visible in Sahara, 
Abyssinia, and Somali). Already in 1919 this 
totality was 6 minutes 50 seconds in duration; in 
1937, as already mentioned, it will be 7 minutes 20 
seconds, and at the subsequent returns even longer 
yet, approaching the estimated maximum of 7 
minutes 58 seconds which has never been observed. 
This remarkable series of total echpses is longer in 
duration than any others during a thousand years. 
Its next subsequent return is in 1991, occurring with 
the eclipsed sun practically at noon in the zenith of 
Mount Popocatepetl in Mexico. 

Whatever may be the progress of solar research 
during the intervening years, it is impossible to 
imagine the alert astronomer of that remote day 
without incentive for further investigation of the 
sun^s corona, in which are concealed no doubt many 
secrets of the sun's evolution from nebula to star^ 



CHAPTER XXXI 

THE SOLAR CORONA 

AND what is the sun*s corona?" mildly asked 
- a college professor of a student who might 
better have answered "Not prepared." 

"I did know, Professor, but I have forgotten," 
was his reply. 

"What an incalculable loss to science," returned 
the professor with a twinkle. "The only man who 
ever knew what the sun's corona is, and he has 
forgotten!" 

Only in part has the mystery of the corona been 
cleared by the research of the present day. Our 
knowledge proceeds but slowly, because the corona 
has never been seen except during total eclipses of 
the sun ; and astronomers, as a matter of fact, have 
never had a fair chance at it. Two total eclipses 
happen on the average of every three years; their 
average duration is only two or three minutes; 
totality can be seen only in a narrow path about 
a hundred miles wide, though it may be several 
thousand miles long; there is usually about equal 
chance of cloud with clear skies; and fully three- 
fourths of the totality areas of the globe are un- 
available because covered by water. So that even 
if we imagine the tracks of eclipses quite thickly 
populated with astronomers and telescopes, at least 
one every hundred miles, how much solid watching 
of the corona would this permit? Only a little more 
than one week's time in a whole century. 

219 



220 ASTRONOMY TO-DAY 

The true corona is at least a triple phenomenon 
and a very complex one. The photographs reveal 
it much as the eye sees it, v^ith all its complexity 
of interlacing streamers projected into a flat, or 
plane, surrounding the disk of the dark moon v^hich 
hides the true sun completely. But we must keep 
in mind the fact that the sun is a globe, not a 
disk, and that the streamers of the corona radiate 
more or less from all parts of the surface of the 
solar sphere, much as quills from a porcupine. 

From the sun's magnetic poles branch out the 
polar rays, nearly straight throughout their visible 
extent. Gradually as the coronal rays originate at 
points around the solar disk farther and farther 
removed from the poles, they are more and more 
curved. Very probably they extend into the equa- 
torial regions, but it is not easy to trace them there 
because they are projected upon and confused with 
the filaments having their origin remote from the 
poles. Then there is the inner equatorial corona, 
apparently connected intimately with truly solar 
phenomena, quite as the polar rays are. The third 
element in the composite is the outer ecliptic co- 
rona, for the most part made up of long streamers. 
This is most fully developed at the time of the few- 
est spots on the sun. It is traceable much farther 
against the black sky with the naked eye than by 
photography. Without any doubt it is a solar ap- 
pendage and possibly it may merge into the zodi- 
acal light. 

Naturally this superb spectacle must have been 
an amazing sight to the beholders of antiquity who 
were fortunate enough to see it. Historical refer- 
ences are rare: perhaps the earliest was by Plu- 
tarch about A. D. 100, who wrote of it, "A radiance 



THE SOLAR CORONA 221 

shone round the rim, and would not suffer darkness 
to become deep and intense." Philostratus a century- 
later mentions the death of the emperor Domitian at 
Ephesus as "announced" by a total eclipse. 

Kepler thought the corona was evidence of a 
lunar atmosphere; indeed, it was not until the 
middle of the 19th century that its lack of relation 
to the moon was finally demonstrated. Later ob- 
servers, Wyberd in 1652 and Ulloa, got the im- 
pression that the corona turned round the disk 
catherine-wheel fashion, "like an ignited wheel 
in fireworks, turning on its center." But no later 
observer has reported anything of the sort. Quite 
the contrary, there it stands against the black sky 
in motionless magnificence a colorless pearly mass 
of wisps and streamers for the most part nebulous 
and ill-defined, fading out very irregularly into the 
black sky beyond, but with a complex interlacing of 
filaments, sometimes very sharply defined near the 
solar poles. It defies the skill of artist and draughts- 
man to sketch it before it is gone. 

Photograph it? Yes, but there are troubles. Of 
course the camera work is superior to sketches by 
hand. As Langley used to say, "The camera has 
no nerves, and what it sets down we may rely on." 
Foremost among the photographic difficulties is the 
wide variation in intensity of the coronal light in 
different regions of the corona. If a plate is ex- 
posed long enough to get the outer corona, the 
exceeding brightness of the inner corona overex- 
poses and burns out that part of the plate or film. 
If the exposure is short, we get certain regions of 
the inner corona excellently, but the outer regions 
are a blank because they can be caught only by a 
long exposure. 



222 ASTRONOMY TO-DAY 

So the only way is to take a series of pictures 
with a wide range of exposures, and then by care- 
ful and artistic handwork, combine them all into 
a single drawing. Wesley of London has succeeded 
eminently in work of this character, and his draw- 
ings of the sun's corona, visible at total eclipses 
from 1871 onward, in possession of the Royal 
Astronomical Society, are the finest in existence. 
They give a vastly better idea of the corona, as 
the eye sees it, than any single photograph possi- 
bly can. 

The early observers apparently never thought of 
the corona as being connected with the sun. It was 
a halo merely, and so drawn. Its real structure was 
neither known, depicted, or investigated. Sketches 
were structureless, as any aureola formed by stray 
sunlight grazing the moon might naturally be. That 
the rays are curved and far from radial round the 
sun was shown for the first time in the sketches 
of 1842, and in 1860 Sir Francis Galton observed 
that the long arms or streamers "do not radiate 
strictly from the center.'' 

The inner corona had first been recorded photo- 
graphically on a daguerreotype plate during the 
eclipse of 1851, but the lens belonged to a heliom- 
eter, and was of course uncorrected for the photo- 
graphic rays. The wet collodion plates of the 
eclipse of 1860, by De la Rue, proved that not only 
the prominences but the corona were truly solar, 
because his series of technically perfect pictures 
revealed the steady and unchanged character of 
these phenomena while the moon's disk was pass- 
ing over them as totality progressed. And at the 
eclipse of 1869, Young put the solar theory of the 
corona beyond the shadov/ of any further doubt 



•THE SOLAR CORONA 223 

by examination of its light with the spectroscope 
and discovering a green line in the spectrum due 
to incandescent vapor of a substance not then 
identified with anything terrestrial, and therefore 
called coronium. 

The total brilliance of the corona was very dif- 
ferently estimated by the earlier observers, though 
pretty carefully measured at later eclipses. The 
standard full moon is used for reference, and at 
one eclipse the corona falls short of, while at an- 
other it will exceed the full moon in brightness. 
Variations in brilliancy are quite marked: at one 
eclipse it was nearly four times as bright as the 
full moon. Much evidence has already accumulated 
on this question; but whether the observed varia- 
tions are real, or due mainly to the varying rela- 
tive sizes of sun and moon at different eclipses, is 
not yet known. The coronal light is largely bluish 
in tint, and this is the region of the spectrum most 
powerfully absorbed by our atmosphere. Eclipses 
are observed by different expeditions located at 
stations where the eclipsed sun stands at very dif- 
ferent altitudes above the horizon; besides this the 
localities of observation are at varied elevations 
above sea level; so that the varying amount of 
absorption of the coronal light renders the prob- 
lem one of much difficulty. 

The long ecliptic streamers of the corona were 
first seen by Newcomb and Langley during the 
totality of 1878. On one side of the sun there was 
a stupendous extension of at least twelve solar 
diameters, or nearly 11 millions of miles. Lang- 
ley observed from the summit of Pike's Peak, over 
14,000 feet high, and was sure that he was wit- 
nessing a '^real phenomenon heretofore unde- 



224 ASTRONOMY TO-DAY 

scribed." The vast advantage of elevation was ap- 
parent also from the fact that he held the corona 
for more than four minutes after true totality had 
ended. These streamers are characteristic of the 
epoch of minimum spots on the sun, as Ranyard 
first suggested. It was found that this type of 
corona had been recorded also in 1867; and it has 
reappeared in 1889, 1900 and 1911, and will doubt- 
less be visible again in 1922. 

How rapidly the streamers of the corona vary 
is not known. Occasionally an observer reports 
having seen the filaments vibrate rapidly as in the 
aurora borealis, but this is not verified by others 
who saw the same corona perfectly unmoving. Com- 
parisons of photographs taken at widely separate 
stations during the same eclipse have shown that at 
least the corona remained stationary for hours at 
a time. Whether it may be unchanged at the end 
of a day, or a week, or a month, is not known ; be- 
cause no two total eclipses can ever happen nearer 
each other than within an interval of 173 days, or 
one-half of the eclipse year. And usually the inter- 
val between total eclipses is twice or three times 
this period. 

Theories of what the solar corona may be are 
very numerous. The extreme inner corona is per- 
haps in part a sort of gaseous atmosphere of the 
sun, due to matter ejected from the sun, and kept 
in motion by forces of ejection, gravity, and repul- 
sion of some sort. Meteoric matter is likely con- 
cerned in it, and Huggins suggested the debris of 
disintegrating comets. Schuster was in agreement 
with Huggins that the brighter filaments of the 
corona might be due to electric discharges, but it 
seems very unlikely that any single hypothesis can 




Venus, Showing Crescent Phase of the Planet. Venus is the earth's 
nearest neighbor on the side toward the sun. (Fhoto, Yerkes Observatory.) 




Mars, the Planet Next Beyond the Earth. The photograph shows 
one of the white polar caps. The caps are thought to be snow or ice and 
may indicate the existence of atmosphere. (Photo, Yerkes Observatory.) 



THE SOLAR CORONA 225 

completely account for the intricate tracery of so 
complex a phenomenon. 

Elaborate spectroscopic programs have been 
carried out at recent eclipses, affording evidence 
that certain regions are due to incandescent matter 
of lower temperature than the sun's surface. A 
small part of the light of the corona is sunlight re- 
flected from dark particles possibly meteoric, but 
more likely dust particles or fog of some sort. This 
accounts for the weakened solar spectrum with 
Fraunhofer absorption lines, and this part of the 
light is polarized. 

Many have been the attempts to see, or photo- 
graph, the corona without an eclipse. None of 
them has, however, suceeded as yet. Huggins got 
very promising results nearly forty years ago, and 
success was thought to have been reached ; but sub- 
sequent experiments on the Riffelberg in 1884 and 
later convinced him that his results related only to 
a spurious corona. In 1887 the writer made an un- 
successful attempt to visualize the corona from the 
summit of Fujiyama, and Hale tried both optical 
and photographic methods on Pikes Peak in 1893 
without success. He devised later a promising 
method by which the heat of the corona in different 
regions can be measured by the bolometer, and an 
outline corona afterward sketched from these 
results. 

Still another method of attacking the problem 
occurred to the writer in 1919, which has not yet 
been carried out. It would take advantage of re- 
cent advances in aeronautics, and contemplates an 
artificial eclipse in the Upper air by means of a 
black spherical balloon. This would be sent up to 
an altitude of perhaps 40,000 feet, where it would 

Sci. Vol. 2—8 



226 ASTRONOMY TO-DAY 

partake of the motion of the air current in which 
it came to equilibrium. Then a snapshot camera 
would be mounted on an areoplane, in which the 
aviator would ascend to such a height that the bal- 
loon just covered the sun, as the moon does in a 
total eclipse. With the center of the balloon in line 
with the sun's center, he would photograph the 
regions of the sky immediately surrounding the sun, 
against which the corona is projected. As the en- 
tire apparatus would be above more than an entire 
half of the earth's atmosphere, the experiment 
would be well worth the attempt, as pretty much 
everything else has been tried and found wanting. 
Needless to say, the importance of seeing the co- 
rona at regular intervals whenever desired, without 
waiting for eclipses of the sun, remains as insistent 
as ever. 



CHAPTER XXXII 

THE RUDDY PLANET 

MARS is a planet next in order beyond the 
eai'th, and its distance from the sun averages 
141^ million miles. It has a relatively rapid motion 
among the stars, its color is reddish, and, when 
nearest to us, it is perhaps the most conspicuous 
object in the sky. 

Mars appeared to the ancients just as it does to us 
to-day. Aristotle recorded an observation of Mars, 
356 B. C, when the moon passed over the planet, or 
occulted it, as our expression is. Galileo made the 
first observations of Mars with a telescope in 1610, 
and his little instrument was powerful enough to 
enable him to discover that the planet had phases, 
though it did not pass through all the phases that 
Mercury and Venus do. This was obvious from the 
fact that Mars is always at a greater distance from 
the sun than we are, and the phase can only be 
gibbous, or about like the moon when midway be- 
tween full and quarter. 

Many observers in the seventeenth century fol- 
lowed up the planet with such feeble optical power 
as the telescopes of that epoch provided : Fontana 
(who made the first sketch), Riccioli and Bianchini 
in Italy, Cassini in France, Huygens in Holland, and 
later Sir William Herschel in England. 

It was Cassini who first made out the whitish 
spots or polar caps of Mars in 1666, but not until 

227 



228 ASTRONOMY TO-DAY 

after Huygens had noted the fact that Mars turned 
round on an axis in a period but little longer than 
the earth's. Cassini followed it up later with a 
more accurate value; and observations in our own 
day, when combined with these early ones, enable 
us to say that the Martian day is equal to 24 
hours 37 minutes 22.67 seconds, accurate probably 
to the hundredth part of a second. 

When we know that a planet turns round on an 
axis, we know that it has a day. When we know 
the direction of the axis in space or in relation to the 
plane of its path round the sun, we know that it 
has seasons : we can tell their length and when they 
begin and end. It did not take many years of ob- 
servation to prove that the axis round which Mars 
turns is tilted to the plane of its path round the 
sun by an angle practically the same as that at 
which the earth's axis is tilted. So there is the im- 
mediate inference that on Mars the order and per- 
haps the character of the seasons is much the same 
as here on the earth. 

At least two things, however, tend to modify 
them. First, the year of Mars is not 365 days like 
ours, but 687 days. Each of the four seasons on 
Mars, therefore, is proportionally longer than our 
seasons are. Then comes the question of atmos- 
phere — ^how much of an atmosphere does Mars 
really possess in proportion to ours, and how would 
its lesser amount modify the blending of the 
seasons into one another? 

All discussion of Mars and the problems of exis- 
tence of life upon that planet hinge upon the char- 
acter and extent of Martian atmosphere. The 
planet seems never to be covered, as the earth 
usually is, with extensive areas of cloud which to 



THE RUDDY PLANET 229 

an observer in space would completely mask its 
oceans and continents. Nearly all the time Mars 
in his equatorial and temperate zones is quite clear 
of clouds. A few whitish spots are occasionally 
seen to change their form and position in both 
northern and southern latitudes, and they vary^ 
with the progress of the day on Mars, as clouds 
naturally would. But Schiaparelli, perhaps the best 
of all observers, thought them to be not low-lying 
clouds of the nimbus type that would produce rains, 
but rather a veil of fog, or perhaps a temporary 
condensation of vapor, as dew or hoar frost. But 
the strongest argument for an atmosphere is based 
on the temporary darkening or obscuration of well 
known and permanent markings on the surface of 
Mars. These are more or less frequently observed 
and clouds afford the best explanation of their 
occurrence. 

So much for evidence supplied by the telescope 
alone. When, however, we employ the spectroscope 
in conjunction with the telescope, another sort of 
evidence is at hand. Several astronomers have 
reached the conclusion that watery vapor exists in 
the atmosphere of Mars, while other astronomers 
equipped with equal or superior apparatus, and 
under equally favorable or even better conditions, 
have reached the remarkable conclusion that the 
spectra of Mars and the moon are identical in every 
particular. From this we should be led to infer 
that Mars has perhaps no more atmosphere than 
the moon has, that is to say, none whatever that 
present instruments and methods of investigation 
have enabled us to detect. 

What then, shall we conclude? Simply that the 
atmosphere of Mars is neither very dense nor exten- 



230 ASTRONOMY TO-DAY 

sive. Probably its lower strata close to the planet's 
surface are about as dense as the earth's atmosphere 
is at the summits of our highest mountains. 

This conclusion is not unwelcome, if we keep a 
few fundamental facts in clear and constant view. 
Mars is a planet of intermediate size betwen the 
earth and the moon: twice the moon's diameter 
(2,160 miles) very nearly equals the diameter of 
Mars (4,200 miles), and twice the diameter of 
Mars does not greatly exceed the earth's diameter 
(7,920 miles). As to the weights or masses of 
these bodies. Mars is about one-ninth, and the moon 
one-eightieth of the earth. The atmospheric en- 
velope of the earth is abundant, the moon has none 
as far as we can ascertain; so it seems safe to 
infer that Mars has an atmosphere of slight den- 
sity: not dense enough to be detected by spectro- 
scopic methods, but yet dense enough to enable us 
to explain the varying telescopic phenomena of the 
planet's disk which we should not know how to ac- 
count for, if there were no atmosphere whatever. 
One astronomer has, indeed, gone so far as to cal- 
culate that in comparison with our planet Mars is 
entitled to one-twentieth as much atmosphere as 
we have, and that the mercurial barometer at "sea 
level" would run about five and a half inches, as 
against thirty inches on the earth. 

In general, then, the climate of Mars is prob- 
ably very much like that of a clear season on a very 
high terrestrial table land or mountain — a climate 
of wide extremes, with great changes of tempera- 
ture from day to night. The inequality of Martian 
seasons is such that in his northern hemisphere the 
winter lasts 381 days and the summer only 306 
days. 



THE RUDDY PLANET 231 

Now, the polar caps of Mars, which are reasonably- 
assumed to be due to snow or hoar frost, attain 
their maximum three or four months after the 
winter solstice, and their minimum about the same 
length of time after the summer solstice. This 
lagging should be interpreted as an argument for a 
Martian atm.osphere with heat-storing qualities, 
similar to that possessed by the earth. 

Upon this characteristic, indeed, depends the 
climate at the surface of Mars : v/hether it is at all 
similar to our own, and whether fluid water is a 
possibility on Mars or not. While the cosmic rela- 
tions of the planet in its orbit are quite the same as 
ours, nevertheless the greater distance of Mars 
diminishes his supply of direct solar heat to about 
half what we receive. On the other hand, his dis- 
tance from the sun during his year of motion 
around it varies much more widely than ours, so 
that he receives when nearest the sun about one-half 
more of solar heat than he does vv^hen farthest away. 

Southern summers on Mars, therefore, must be 
much hotter, and southern winters colder than the 
corresponding seasons of his northern hemisphere. 
Indeed, the length of the southern sum.mer, nearly 
twice that of the terrestrial season, sometimes 
amply suffices to melt all the polar ice and snow, 
as in October, 1894, when the southern polar cap 
of Mars dwindled rapidly and finally vanished 
completely. 

Very interesting in this connection are the re- 
searches of Stoney on the general conditions affect- 
ing planetary atmospheres and their composition. 
According to the kinetic theory, if the molecules of 
gases which are continually in motion travel out- 
ward from the center of a planet, as they fre- 



232 ASTRONOMY TO-DAY 

quently must, and with velocities surpassing the 
limit that a planet's gravity is capable of control- 
ling, these molecules will effect a permanent 
escape from the planet, and travel through space in 
orbits of their own. 

So the moon is wholly without atmosphere 
because the moon's gravity is not powerful enough 
to retain the molecules of its component gases. So 
also the earth's atmosphere contains no helium or 
free hydrogen. So, too. Mars is possessed of insuf- 
ficient force of gravity to retain water vapor, and 
the Martian atmosphere may therefore consist 
mainly of nitrogen, argon, and carbon dioxide. 

As everyone knows, the axis of the earth if ex- 
tended to the northern heavens would pass very 
near the north polar star, which on that account is 
known as Polaris. In a similar manner the axis 
of Mars pierces the northern heavens about mid- 
way between the two bright stars Alpha Cephei 
and Alpha Cygni (Deneb). The direction of this 
axis is pretty accurately known, because the meas- 
urement of the polar caps of the planet as they turn 
round from night to night, year in and year out, has 
enabled astronomers to assign the inclination of the 
axis with great precision. 

These caps are a brilliant white, and they are 
generally supposed to be snow and ice. They 
wax and wane alternately with the seasons on 
Mars, being largest at the end of the Martian win- 
ter and smallest near the end of summer. The 
existence of the polar caps together with their 
seasonal fluctuations afford a most convincing 
argument for the reality of a Martian atmosphere, 
sufficiently dense to be capable of diffusing and 
transporting vapor. 



THE RUDDY PLANET 233 

The northern cap is centered on the pole almost 
with geometric exactness, and as far as the 85th 
parallel of latitude. On the other hand, the south 
polar cap is centered about 200 miles from the true 
pole, and this distance has been observed to vary 
from one season to another. No suggestion has been, 
m.ade to account for this singular variation. On one 
occasion it stretched down to Martian latitude 70 
degrees and was over 1,200 miles in diameter. 

Pickering watched the clianging conditions of 
shrinking of the south polar cap in 1892 with a 
large telescope located in the Andes of Peru. Mars 
was faithfully followed on every night but one from 
July 13 to September 9, and the apparent altera- 
tions in this cap were very marked, even from night 
to night. As the snows began to decrease, a long 
dark line made its appearance near the middle of 
the cap, and gradually grew until it cut the cap in 
two. This white polar area (and probably also the 
northern one in similar fashion) becomes notched 
on the edge with the progress of its summer sea- 
son; dark interior spots and fissures form, isolated 
patches separate from the principal mass, and later 
seem to dissolve and disappear. Possibly if one 
were located on Mars and viewing our earth with a 
big telescope, the seasonal variation of our north 
and south polar caps might present somewhat simi- 
lar phenomena. All the recent oppositions of Mars 
have been critically observed by Pickering from an 
excellent station in Jamaica. 

Quite obviously the fluctuations of the polar caps 
are the key to the physiographic situation on Mars, 
and they are made the subject of the closest 
scrutiny at every recurring opposition of the planet. 
Several observers, Lowell in particular, record a 



234 ASTRONOMY TO-DAY 

bluish line or a sort of retreating polar sea, follow- 
ing up the diminishing polar cap as it shrinks with 
the advance of summer. It is said that no such line 
is visible during the formation of the polar cap with 
the approach of winter. All such results of critical 
observation^ just on the limit of visibility, have to 
be repeated over and over again before they become 
part of the body of accepted scientific fact. And in 
many instances the only sure way is to fall back on 
the photographic record, which all astronomers, 
whether prejudiced or not, may have the opportunity 
to examine and draw their individual conclusions. 
Already the approaching opposition of 1924, the 
most favorable since the invention of the telescope, 
is beginning to attract attention, and preparations 
are in progress, of new and more powerful instru- 
ments, with new and more sensitive photographic 
processes, by means of which many of the present 
riddles of Mars may be solved. 



CHAPTER XXXIII 

THE CANALS OF MARS 

THEN there are the so-called canals of Mars, 
about which so much is written and relatively 
little known. Faint markings which resemble them 
in character were first drawn in 1840 and later in 
1864, but Schiaparelli, the famous Italian astrono- 
mer, is probably their original discoverer, when 
Mars was at its least distance from the earth in 
1877. He made the first accurate detailed map of 
Mars at this time, and most of the important or 
more conspicuous canals {canali, he called them in 
Italian, that is, channels merely, without any refer- 
ence whatever to their being watercourses) were 
accurately charted by him. 

At all the subsequent close approaches of Mars, 
the canals have been critically studied by a wide 
range of astronomical observers, and their conclu- 
sions as to the nature and visibility of the canals 
have been equally wide and varied. The most 
favorable oppositions have occurred in 1892 and 
1894, also in 1907 and 1909. On these occasions 
a close minimum distance of Mars was reached, that 
is, about 35 millions of miles ; but in 1924 the planet 
makes the closest approach in a period of nearly a 
thousand years. Its distance will not much exceed 
30 millions of miles. 

But although this is a minimum distance for Mars, 
it must not be forgotten that it is a really vast 

235 



236 ASTRONOMY TO-DAY 

distance, absolutely speaking; it is something like 
150 times greater than the distance of the moon. 
With no telescopic power at our command could we 
possibly see anything on the moon of the size of 
the largest buildings or other works of human in- 
telligence; so that we seem forever barred from 
detecting anything of the sort on Mars. 

Nevertheless, the closest scrutiny of the ruddy 
planet by observers of great enthusiasm and intelli- 
gence, coupled with imagination and persistence, 
have built up a system of canals on Mars, covering 
the surface of the planet like spider webs over 
a printed page, crossing each other at intersect- 
ing spots known as "lakes," and embodying a 
wealth of detail which challenges criticism and 
explanation. 

To see the canals at all requires a favorable 
presentation of Mars, a steady atmosphere and a 
perfect telescope, with a trained eye behind it. Not 
even then are they sure to be visible. The training 
of the eye has no doubt much to do with it. So 
photography has been called in, and very excellent 
pictures of Mars have already been taken, some 
nearly half as large as a dime, showing plainly the 
lights and shades of the grander divisions of the 
Martian surface, but only in a few instances reveal- 
ing the actual canals more unmistakably than they 
are seen at the eyepiece. 

The appearance and degree of visibility of the 
canals are variable: possibly clouds temporarily 
obscure them. But there is a certain capriciousness 
about their visibility that is little understood. In 
consequence of the changing physical aspects, as to 
season, on Mars and his orbital position with ref- 
erence to the earth, some of the canals remain for 



THE CANALS OF MARS 237 

a long time invisible, adding to the intricacy of the 
puzzle. 

For the most part the canals are straight in their 
course and do not swerve much from a great circle 
on the planet. But their lengths are very different, 
some as short as 250 miles, some as long as 4,000 
miles; and they often join one another like spokes 
in the hub of a wheel, though at various angles. As 
depicted by Lowell and his corps of observers at 
Flagstaff, Arizona, the canal system is a truly mar- 
velous network of fine darkish stripes. Their color 
is represented as a bluish green. 

Each marking maintains its own breadth through- 
out its entire length, but the breadth of all the canals 
is by no means the same : the narrowest are perhaps 
fifteen to twenty miles wide, and the broadest prob- 
ably ten times that. At least that must be the 
breadth of the Nilosyrtis, which is generally re- 
garded as the most conspicuous of all the canals. The 
Lowell Observatory has outstripped all others in the 
number of canals seen and charted, now about 500. 

What may be the true significance of this remark- 
able system of markings it is impossible to conclude 
at present. Schiaparelli from his long and critical 
study of them, their changes of width and color, was 
led to think that they may be a veritable hydro- 
graphic system for distributing the liquid from the 
melting polar snows. In this case it would be diffi- 
cult to escape the conviction that the canals have, at 
least in part, been designed and executed with a 
definite end in view. 

Lowell went even farther and built upon their 
behavior an elaborate theory of life on the planet, 
with intelligent beings constructing and opening 
new canals on Mars at the present epoch. Pickering 



238 ASTRONOMY TO-DAY 

propounded the theory that the canals are not water- 
bearing channels at all, but that they are due to 
vegetation, starting in the spring when first seen 
and vitalized by the progress of the season poleward, 
the intensity of color of the vegetation coinciding 
with the progress of the season as we observe it. 

Extensive irrigation schemes for conducting 
agricultural operations on a large scale seem a very 
plausible explanation of the canals, especially if we 
regard Mars as a world farther advanced in its life 
history than our own. Erosion may have worn the 
continents down to their minimum elevation, ren- 
dering artificial waterways not difficult to build; 
while with the vanishing Martian atmosphere and 
absence of rains, the necessity of water for the sup- 
port of animal and vegetal life could only be met 
by conducting it in artificial channels from one re- 
gion of the planet to another. 

Interesting as this speculative interpretation is, 
however, we cannot pass by the fact that many com- 
petent astronomers with excellent instruments finely 
located have been unable to see the canals, and there- 
fore think the astronomers who do see them are 
deceived in some way. Also many other astrono- 
mers, perhaps on insufficient grounds, deny their 
existence in toto. 

Many patient years of labor would be required to 
consult all the literature of investigation of the 
planet Mars, but much of the detail has been 
critically embodied in maps at different epochs, by 
Kayser, Proctor, Green, and Dreyer. And Flam- 
marion in two classic volumes on Mars has pre- 
sented all the observations from the earliest time, 
together with his own interpretation of them. Areo- 
graphy is a term sometimes applied to a description 



THE CANALS OF MARS 239 

of the surface of Mars, and it is scarcely an ex- 
aggeration to say that areography is now better 
known than the geography of immense tracts of the 
earth. 

For some reason well recognized, though not at all 
well understood, Mars although the nearest of all 
the planets, Venus alone excepted, is an object by 
no means easy to observe with the telescope. Pos- 
sibly its unusual tint has something to do with this. 
With an ordinary opera glass examine the moon 
very closely, and try to settle precise markings, 
colors, and the nature of objects on her surface; 
Mars under the best conditions, scrutinized with 
our largest and best telescope, presents a problem 
of about the same order of difficulty. There are 
delicate and changing local colors that add much 
uncertainty. Nevertheless, the planet's leading 
features are well made out, and their stability since 
the time of the earliest observers leaves no room to 
doubt their reality as parts of a permanent plane- 
tary crust. 

The border of the Martian disk is brighter than 
the interior, but this brightness is far from uniform. 
Variations in the color of the markings often depend 
on the planet's turning round on its axis, and the 
relation of the surface to our angle of vision. If 
we keep in mind these obstacles to perfect vision in 
our own day, it is easy to see why the early users of 
very imperfect telescopes failed to see very much, 
and were misled by much that they thought they 
saw. Then, too, they had to contend, as we do, with 
unsteadiness of atmosphere, which is least trouble- 
some near the zenith. 

As their telescopes were all located in the north- 
ern hemisphere, the northern hemisphere of Mars 



240 ASTRONOMY TO-DAY 

is the one best circumstanced for their investiga- 
tion; because at the remote oppositions of Mars, 
which always happen in our northern winter with 
the planet in high north declination, it is always the 
north pole of Mars which is presented to our 
view. Whereas the close oppositions of the planet 
always come in our northern midsummer, with 
Mars in south declination and therefore passing 
through the zenith of places in corresponding south 
latitude. 

With Mars near opposition, high up from the 
horizon, a fairly steady atmosphere, and a magnify- 
ing power of at least 200 diameters, even the most 
casual observer could not fail to notice the striking 
difference in brightness of the two hemispheres: 
the northern chiefly bright and the southern 
markedly dark. Formerly this was thought to indi- 
cate that the southern hemisphere of Mars was 
chiefly water and the northern land, much as is the 
case on the earth: with this difl^erence, however, 
that water and land on the earth are proportioned 
about as eleven to four. 

But Mars in its general topography presents no 
analogy with the present relation of land and water 
on the earth. There seems no reason to doubt that 
the northern regions with their prevailing orange 
tint, in some places a dark red and in others fading 
to yellow and white, are really continental in charac- 
ter. Other vast regions of the Martian surface are 
possibly marshy, the varying depth of water causing 
the diversity of color. If we could ever catch a 
reflection of sunlight from any part of the surface 
of Mars, we might conclude that deep water exists 
on the planet; but the farther research progresses, 
the more complete becomes the evidence that per- 



THE CANALS OF MARS 241 

manent water areas on Mars, if they exist at all, 
are extremely limited. 

Since 1877 Mars has been known to possess two 
satellites, which were discovered in August of that 
year by Hall at Washington. Moons of this planet 
had long been suspected to exist and on one or two 
previous occasions critically looked for, though 
without success. In the writings of Dean Swift 
there is a fanciful allusion to the two moons of 
Mars; and if astronomers had chanced to give 
serious attention to this, Phobos and Deimos, as 
Hale named them, might have been discovered long 
before. 

They are very small bodies, not only faint in the 
telescope, but actually of only ten or twenty miles 
diameter; and from the strange relation that 
Phobos, the inner moon, moves round Mars three 
times while the planet itself is turning round only 
once on its axis, some astronomers incline to the 
hypothesis that this moon at least was never part of 
Mars itself, but that it was originally an inner or 
very eccentric member of the asteroid group, which 
ventured within the sphere of gravitation of Mars, 
was captured by that planet, and has ever since 
been tributary to it as a secondary body or satellite. 



CHAPTER XXXIV 

LIFE IN OTHER WORLDS 

POPULAR interest in astronomy is exceedingly 
wide, but it is very largely confined to the idea of 
resemblances and differences between our earth 
and the bodies of the sky. The question most 
frequently asked the astronomer is, "Have any of 
the stars got people on them T' Or more specifically, 
"Is Mars inhabited?" The average questioner will 
not readily be turned off with yes or no for an 
answer. He may or may not know that it is quite 
impossible for astronomers to ascertain anything 
definite in this matter, most interesting as it is. 
What he wants to find out is the view of the in- 
dividual astronomer on this absorbing and ever re- 
curring inquiry. 

We ought first to understand what is meant by 
the manifestation here on the earth called life, and 
agree concerning the conditions that render it 
possible. Apparently they are very simple. We 
may or may not agree that a counterpart of life, or 
life of a wholly different type from ours, may exist 
on other planets under conditions wholly diverse 
from those recognized as essential to its existence 
here. The problem of the origin of life is, in the 
present state of knowledge, highly speculative and 
hardly within the domain of science. Here on 
earth, life is intimately associated with certain 
chemical compounds, in which carbon is the common 

242 



LIFE IN OTHER WORLDS 243 

element without which life would not exist. Also 
hydrogen, oxygen, and nitrogen are present, with 
iron, sulphur, phosphorus, magnesium and a few 
less important elements besides. But carbon is the 
only substance absolutely essential. Protoplasm 
cannot be built without it, and protoplasm makes up 
the most of the living cell. Closely related to car- 
bon is silica also, as a substitution in certain organic 
compounds. Protoplasm is able to stand very low 
temperatures, but its properties as a living cell cease 
when the temperature reaches 150 Fahrenheit. 

Animal life as it exists on the earth to-day ap- 
pears to have been here many million years. The 
palseontologists agree that all life originated in the 
waters of the earth. It has passed through evolu- 
tionary stages from the lowest to the highest. 
Throughout this vast period the astronomer is able 
to say that the conditions of the earth which 
appear to be essential to the maintenance of life 
have been pretty constantly what they are to-day. 
The higher the type of life, the narrower the range 
of conditions under which it thrives. Man can 
exist at the frigid poles even if the temperature is 
75 degrees below Fahrenheit zero; and in the 
deserts and the tropics, he swelters under temper- 
atures of 115 degrees, but he still lives. At these 
extremes, however, he can scarcely be said to thrive. 

We have, then, a relatively narrow range of tem- 
peratures which seems to be essential to his com- 
fortable existence and development : we may call it 
150 degrees in extent. Had not the surface temper- 
ature of the earth been maintained within this range 
for indefinite ages, in the regions where the human 
race has developed, quite certainly man would not be 
here. How this equability of temperature has been 



244 ASTRONOMY TO-DAY 

maintained does not now matter. Clearly the earth 
must have existed through indefinite ages in the 
process of cooling down from temperatures of at 
least 6,000 degrees. 

During this stage the temperature of the surface 
was earth-controlled. Then this period merged 
very gradually into the stage where life became 
possible, and the temperature of the surface became^ 
as it now is, sun-controlled. How many years are 
embraced in this span of periods, or ages, we have 
no means of knowing. But of the sequence of 
periods and the secular diminution of temperature, 
we may be certain. 

Then there is the equally important considera- 
tion of water necessary for the origination, support, 
and development of life. We cannot conceive of 
life existing without it. On the earth water is 
superabundant, and has been for indefinite ages in 
the past. There is little evidence that the oceans 
are drying up; although the commonly accepted 
view is that the waters of the earth will very gradu- 
ally disappear. Water can exist in the fluid state, 
which is essential to life, at all temperatures be- 
tween 32 degrees and 680 degrees F. 

Air to breathe is essential to life also. The atmos- 
phere which envelops the earth is at least 100 miles 
in depth, and its own weight compresses it to a 
tension of nearly 15 pounds to the square inch at 
sea level. This atmosphere and its physical prop- 
erties have had everything to do with the develop- 
ment of animal life on the planet. Without it and 
its remarkable property of selective absorption, 
which imprisons and diffuses the solar heat, it is in- 
conceivable that the necessary equability of surface 
temperature could be maintained. This appears to 



LIFE IN OTHER WORLDS 245 

be quite independent of the chemical constituents of 
the atmosphere, and is perhaps the most important 
single consideration affecting the existence of life 
on a planet. If the surface of a planet is partly 
covered with water, it will possess also an atmos- 
phere containing aqueous vapor. 

Heat, water, and air : these three essentials deter- 
mine whether there is life on a planet or not. Of 
course there must be nutrition suitable to the or- 
ganism; mineral for the vegetal, and vegetal for 
the animal. But the narrow range of variation 
appears to be the striking thing: relatively but a 
few degrees of temperature, and a narrow margin of 
atmospheric pressure. If this pressure is doubled 
or trebled, as in submarine caissons, life becomes in- 
supportable. If, on the other hand, it is reduced 
even one-third, as on mountains even 13,000 feet 
high, the human mechanism fails to function, partly 
from lack of oxygen necessary in vitalizing the 
blood, but mainly because of simple reduction of 
mechanical pressure. 

If, then, we conceive of life in other worlds and 
it is agreed that life there must manifest itself much 
as it does here, our answer to the question of habit- 
ability of the planets must follow upon an investiga- 
tion of what we know, or can reasonably surmise, 
about the surface temperatures of these bodies, 
whether they have water, and what are the probable 
physical characteristics of their atmospheres. 

We may inquire about each planet, then, concern- 
ing each of these details. 

The case of Mercury is not difficult. At an 
average distance of only 36 million miles from the 
sun, and with a large eccentricity of orbit which 
brings it a fifth part nearer, conditions of tempera- 



246 ASTRONOMY TO-DAY 

ture alone must be such as to forbid the existence of 
life. The solar heat received is seven times greater 
than at the earthy and this is perhaps sufficient 
reason for a minimum of atmosphere, as indicated 
by observation. If no air, then quite certainly no 
water, as evaporation would supply a slight atmos- 
phere. But according to the kinetic theory of gases, 
the mass of Mercury, only a very small fraction of 
that of the sun, is inadequate to retain an atmos- 
pheric envelope. If, however, the planet's day and 
year are equal, so that it turns a constant face to 
the sun, surface conditions would be greatly com- 
plicated, so that we cannot regard the planet as 
absolutely uninhabitable on the hemisphere that is 
always turned away from the sun. 

Venus at 67 millions of miles from the sun 
presents conditions that are quite different. She 
receives double the solar heat that we do, but pos- 
sessijig an atmosphere perhaps threefold denser 
than ours, as reliably indicated by observations of 
transits of Venus, the intensity of the heat and its 
diffusion may be greatly modified. What the selec- 
tive absorption of the atmosphere of Venus may be, 
we do not know. Nor is the rotation time of the 
planet definitely ascertained: if equal to her year, 
as many observations show and as indicated by the 
theory of tidal evolution, there may well be certain 
regions on the hemisphere perpetually turned away 
from the sun where temperature conditions are 
identical with those on the tropical earth, and where 
every condition for the origin and development of 
life is more fully met than anywhere else in the 
solar system. Whether Venus has water distributed 
as on the earth we do not know, as her surface is 
never seen, owing to dense clouds under which she 



LIFE IN OTHER WORLDS 247 

is always enshrouded. Her cloudy condition pos- 
sibly indicates an overplus of water. 

Is the moon inhabited? Quite certainly not: no 
appreciable air, no water, and a surface tempera- 
ture unmodified by atmosphere — rising perhaps to 
100 degrees F. during the day, which is a fortnight 
in length, and falling at night to 300 degrees below 
zero, if not lower. 

Is Mars inhabited? The probable surface tem- 
perature is much lower than the earth's, because 
Mars receives only half as much solar heat as we do ; 
and more important still, the atmosphere of Mars is 
neither so dense nor so extensive as our own. 
Seasons on Mars are established, much the same as 
here, except that they are nearly twice as long as 
ours ; and alternate shrinking and enlarging of the 
polar caps keeps even pace with the seasons, there- 
by indicating a certainty of atmosphere whose 
equatorial and polar circulation transports the 
moisture poleward to form the snow and ice of 
which the polar caps no doubt consist. 

There is a variety of evidence pointing to an at- 
mosphere on Mars of one-third to one-half the den- 
sity of our own: an atmosphere in which free 
hydrogen could not exist, although other gases 
might. The spectroscopic evidence of water vapor 
in the Martian atmosphere is not very strong. It 
is very doubtful whether water exists on Mars in 
large bodies: quite certainly not as oceans, though 
the evidence of many small "lakes" is pretty well 
made out. With very little water, a thin atmos- 
phere and a zero temperature, is Mars likely to be 
inhabited at the present time? The chances are 
rather against it. If, however, the past develop- 
ment of the planet has progressed in the way usually 



248 ASTRONOMY TO-DAY 

considered as probable, we may be practically cer- 
tain that Mars has been inhabited in the past, when 
water was more abundant, and the atmosphere more 
dense so as to retain and diffuse the solar heat. 

Biologists tell me that they hardly know enough 
regarding the extreme adaptability of organisms 
to environment to enable them to say whether life 
on such a planet as Mars would or would not keep 
on functioning with secular changes of moisture and 
temperature. The survival of a race might be in- 
sured against extremely low temperatures by dwell- 
ing in sub-Martian caves, and sufficient water might 
be preserved by conceivable engineering and me- 
chanical schemes; but the secular reduction of the 
quantity and pressure of atmosphere — it is not easy 
to see how a race even more advanced than ourselves 
could maintain itself alive under serious lack of an 
element so vital to existence. Both Wallace, the 
great biologist, and Arrhenius, the eminent chemist 
(but biologist, astronomer, and physicist as well), 
both reject the habitation theory of Mars, regard- 
ing the so-called canals as quite like the luminous 
streaks on the moon ; that is, cracks in the volcanic 
crust caused by internal strains due to the heated 
interior. Wallace, indeed, argues that the planet is 
absolutely uninhabitable. 

The asteroids, or minor planets? We may dis- 
miss them with the simple consideration that their 
individual masses are so insignificant and their 
gravity so slight that no atmosphere can possibly 
surround them. Their temperatures must be ex- 
ceedingly low, and water, if present at all, can only 
exist in the form of ice. 

Jupiter, the giant planet, presents the opposite 
extreme. His mass is nearly a thousandth "part of 



LIFE IN OTHER WORLDS 249 

the sun's, and is sufficient to retain a very high 
temperature, probably approximating to the condi- 
tion we call red-hot. This precludes the possibility 
of life at the outset, although the indications of a 
very dense atmosphere many thousand miles in 
depth are unmistakable. 

Of Saturn, one thirty-five hundredth the mass of 
the sun, practically the same may be said. Proctor 
thought it quite likely that Saturn might be habit- 
able for living creatures of some sort, but he re- 
garded the planet as on many accounts unsuitable 
as a habitation for beings constituted like ourselves. 
Mere consideration of surface temperature pre- 
cludes the possibility of life in the present stage of 
Saturn's development ; but the consensus of opinion 
is to the effect that life may make its appearance on 
these great planets at some inconceivably remote 
epoch in the future when the surface temperature is 
sufficiently reduced for life processes to begin. Dis- 
coveries of algse flourishing in hot springs approach- 
ing 200 degrees Fahrenheit makes it possible that 
these beginnings may take place earlier and at 
much higher temperatures than have hitherto been 
thought possible. 

A century ago, when the ring of Saturn was 
believed to be a continuous plane, this was a favor- 
ite corner of the solar system for speculation as to 
habitability ; but now that we know the true con- 
stitution of the rings, no one would for a moment 
consider any such possibility. Conditions may, 
however, be quite different with Saturn's huge 
satellite Titan, the giant moon of the solar system. 
Its diameter makes it approximately the size of the 
planet Mars; and although it is much farther re- 
moved from the sun, its relative nearness to the 



250 ASTRONOMY TO-DAY 

highly heated globe of Saturn may provide that 
equability of temperature which is essential to life 
processes. 

Also the three inner Galilean moons of Jupiter, 
especially III which is about the size of Titan, are 
excellently placed for life possibilities, as far as 
probable temperature is concerned, but we have of 
course no basis for surmising what their conditions 
may be as to air and water, except that their small 
mass would indicate a probable deficiency of those 
elements. 

Uranus and Neptune are planets so remote, and 
their apparent disks are so small, that very little is 
known about their physical condition. They are 
each about one-third the diameter of Jupiter, and 
the spectrum of Uranus shows broad diffused bands, 
indicating strong absorption by a dense atmosphere 
very different from that of the earth. Indications 
are that Neptune has a similar atmosphere. 

It is possible that the denser atmospheres of these 
remote planets may be so conditioned as to selective 
absorption that the relatively slender supply of solar 
heat may be conserved, and thus insure a relatively 
high surface temperature v/hen the sun comes into 
control. If our theories of origin of the planets are 
to be trusted, we may rather suppose that Uranus 
and Neptune are still in a highly heated condition; 
that life has not yet made its appearance on them, 
but that it will begin its development ages before 
Saturn and Jupiter have cooled to the requisite tem- 
perature. 

Comets? In his Lettres Cosmologiques (1765) 
Lambert considers the question of habitability of 
the comets, naturally enough in his day, because he 
thought them solid bodies surrounded by atmos- 



LIFE IN OTHER WORLDS 251 

phere, and related to the planets. The extremes of 
temperature at perihelia and aphelia to which comets 
are subjected did not bother him particularly. 

After calculating that the comet of 1680, "being 
160 times nearer to the sun than we are ourselves, 
must have been subjected to a degree of heat 
25,600 times as great as we are," Lambert goes on 
to say : "Whether this comet was of a more compact 
substance than our globe, or was protected in some 
other way, it made its perihelion passage in safety, 
and we may suppose all its inhabitants also passed 
safely. No doubt they would have to be of a more 
vigorous temiperament and of a constitution very 
different from our own. But why should all living 
beings necessarily be constituted like ourselves? Is 
it not infinitely more probable that amongst the 
different globes of the universe a variety of organ- 
izations exist, adapted to the wants of the people 
who inhabit them, and fitting them for the places 
in which they dwell, and the temperatures to which 
they will be subjected? Is man the only inhabitant 
of the earth itself? And if we had never seen 
either bird or fish, should we not believe that the 
air and water were uninhabitable? Are we sure 
that fire has not its invisible inhabitants, whose 
bodies, made of asbestos, are impenetrable to flame ? 
Let us admit that the nature of the beings who 
inhabit comets is unknown to us; but let us not 
deny their existence, and still less the possibility 
of it." 

Little enough is really known about the physical 
nature of comets even now, but what we do know in- 
dicates incessant transformation and instability of 
conditions that would render life of any type ex- 
ceedingly difficult of maintenance. 



252 . ASTRONOMY TO-DAY 

A word about Sir William Herschers theory of 
the sun and its habitability. He thought the core of 
the sun a dark, solid body, quite cold, and sur- 
rounded by a double layer, the inner one of which 
he conceived to act as a sort of fire screen to shield 
the sun proper against the intense heat of the outer 
laj^er, or photosphere by which we see it. Viewed 
in this light, the sun, he says, "appears to be nothing 
else than a very eminent, large and lucid planet, 
evidently the first, or, in strictness of speaking, the 
only primary one of our system .... It is most prob- 
ably also inhabited, like the rest of the planets, by 
beings whose organs are adapted to the peculiar 
circumstances of that vast globe.'' But physics and 
biology were undeveloped sciences in Herschel's days. 

Herschel knew, however, that the stars are all 
suns, so that he must have conceived that they are 
inhabited also, quite independently of the question 
whether they possess retinues of planets, after the 
manner of our solar system. 

This again is a question to which the astronomer 
of the present day can give no certain answer. So 
immensely distant are even the nearest of these 
multitudinous bodies that no telescope can ever be 
built large enough or powerful enough to reveal a 
dark planet as large as Jupiter, alongside even the 
nearest fixed star. Whatever may be the process 
of stellar evolution, there doubtless is an era of 
many hundreds of millions of years in the life of a 
star when it is passing through a planet-maintain- 
ing stage. This would likely depend upon spectral 
type, or to be indicated by it ; and as about half of 
the stars are of the solar type, it would be a reason- 
able inference that at least half of the stars may 
have planets tributary to them. 



LIFE IN OTHER WORLDS 253 

In such a case, the chances must be overwhelm- 
ingly in favor of vast numbers of the planets of 
other stellar systems being favorably circumstanced 
as to heat and moisture for the maintenance of life 
at the present time. That is, they are habitable, 
and if habitable, then thousands of them are no 
doubt inhabited now. But astronomers know abso- 
lutely nothing about this question, nor are they able 
to conceive at present any way that may lead them 
to any definite knowledge of it. There is, indeed, 
one piece of quasi-evidence which might reasonably 
be interpreted as implying that it is more likely 
that the stars are not attended by families of planets 
than that they are. 



CHAPTER XXXV 

THE LITTLE PLANETS 

ALONG toward the end of the eighteenth century 
^ and the beginning of the nineteenth, astrono- 
mers were leading a quiet unexcited life. Sir Wil- 
liam Herschel had been knighted by King George for 
his discovery of the outer planet Uranus, and practi- 
cally everything seemed to be known and discovered 
in the solar system with a single exception. Be- 
tween Mars and Jupiter there existed an obvious 
gap in the planetary brotherhood. 

Could it be possible that some time in the remote 
cosmic past a planet had actually existed there, and 
that some celestial cataclysm had blov/n it to frag- 
ments? If so, would they still be traveling round 
the sun as individual small planets? And might it 
not be possible to discover some of them among the 
faint stars that make up the belt of the zodiac in 
which all the other planets travel ? 

So interesting was this question that the first in- 
ternational association of astronomers banded them- 
selves together to carry on a systematic search 
round the entire zodiacal heavens in the faint hope 
of detecting possible fragments of the original 
planet of mere hypothesis. 

The astronomers of that day placed much reliance 
on what is known as B ode's law — not a law at all, 
but a mere arithmetical succession of numbers 
which represented very well the relative distances 

254 



THE LITTLE PLANETS 255 

of all the planets from the sun. And the distance 
of the newly found Uranus fitted in so well with 
this law that the utter absence of a planet in the gap 
between Mars and Jupiter became very strongly 
marked. 

Quite by accident a discovery of one of the 
guessed-at small planetary bodies was made, on 
January 1, 1801, in Palermo, Sicily, by Piazzi, who 
was regularly occupied in making an extensive cata- 
logue of the stars. His observations soon showed 
that the new object he had seen could not be a fixed 
star, because it moved from night to night among 
the stars. He concluded that it was a planet, and 
named it Ceres (1), for the tutelary goddess of 
Sicily. 

Other astronomers kept up the search, and an- 
other companion planet, Pallas (2) was found in 
the following year. Juno (3) was found in 1804, 
and Vesta (4), the largest and brightest of all the 
minor planets, in 1807. Vesta is sometimes bright 
enough when nearest the earth to be seen with the 
naked eye; but it was the last of the brighter ones, 
and no more discoveries of the kind were made till 
the fifth was found in 1845. Since then discoveries 
have been made in great abundance, more and more 
with every year till the number of little planets at 
present known is very near 1,000. 

The early asteroid hunters found the search 
rather tedious, and the labor increased as it became 
necessary to examine the increasing thousands of 
fainter and fainter stars that must be observed in 
order to detect the undiscovered planets, which 
naturally grow fainter and fainter as the chase is 
prolonged. First a chart of the ecliptic sky had to 
be prepared containing all the stars that the tele- 



256 ASTRONOMY TO-DAY 

scope employed in the search would show. Some of 
the most detailed charts of the sky in existence were 
prepared in connection with this work, particularly 
by the late Dr. Peters of Hamilton College. Once 
such charts are complete, they are compared with 
the sky, night after night when the moon is absent. 
Thousands upon thousands of tedious hours are 
spent in this comparison, with no result whatever 
except that chart and sky are found to correspond 
exactly. 

But now and then the planet hunter is rewarded 
by finding a new object in the sky that does not 
appear on his chart. Almost certainly this is a 
small planet, and only a few night's observation will 
be necessary to enable the discoverer to find out 
approximately the orbit it is traveling in, and 
whether it is out-and-out a new planet or only one 
that had been previously recognized, and then lost 
track of. 

Nearly all the minor planets so far found have 
had names assigned to them principally legendary 
and mythological, and a nearly complete catalogue 
of them, containing the elements of their orbits 
(that is, all the mathematical data that tell us about 
their distance from the sun and the circumstances 
of their motion around him) is published each year in 
the "Annuaire du Bureau des Longitudes" at Paris. 
But these little planets require a great deal of care 
and attention, for some astronomers must accurately 
observe them every few years, and other astrono- 
mers must conduct intricate mathematical computa- 
tions based on these observations; otherwise they 
get lost and have to be discovered all over again. 
Professor Watson, of the University of Michigan 
and later of the University of Wisconsin, endowed 



Jupiter, Largest of the Planets. The irregular belts change th'^ir 

mutual relation and shapes because they do not represent land, but 

are part of the atmosphere. {Photo, Yerkes Observatory.) 



The Planet Neptune and its Satellite. The photograph required 
an exposure of the plate for one hour. {Photo, Yerkes Observatory.) 




Saturn, as Seen Through the 40-inch Refractor, at the time 

when only the edge of the rings is visible, showing condensations. 

{Photo, Yerkes Observatory.) 




Saturn, Photographed Through the 40-inch Refractor. The 
rings appear opened to the fullest extent they can be seen from the 
earth. The picture was made July 7, 1898. {Photo, Yerkes Observa- 
tory. ) 



THE LITTLE PLANETS 257 

the 22 asteroids of his own discovery, leaving to the 
National Academy of Sciences a fund for prosecut- 
ing this work perpetually, and Leuschner is now 
ably conducting it. 

While the number of the asteroids is gratifyingly 
large, their individual size is so small and their total 
mass so slight that, even if there are a hundred 
thousand of them (as is wholly possible) , they would 
not be comparable in magnitude with any one of the 
great planets. Vesta, the largest, is perhaps 400 
miles in diameter, and if composed of substances 
similar to those which make up the earth, its mass 
may be perhaps one twenty-thousandth of the earth's 
mass. If we calculate the surface gravity on such a 
body, we find it about one-thirtieth of what it is 
here; so that a rifle ball, if fired on Vesta with a 
muzzle velocity of only 2,000 feet a second, might 
overmaster the gravity of the little planet entirely 
and be projected in space never to return. 

If, as is likely, some of the smallest asteroids are 
not more than ten miles in diameter, their gravity 
must be so feeble a force that it might be overcome 
by a stone thrown from the hand. There is no re- 
liable evidence that any of the asteroids are sur- 
rounded by atmospheric gases of any sort. Probably 
they are for the most part spherical in form, 
although there is very reliable evidence that a few 
of the asteroids, being variable in the amount of 
sunlight that they reflect, are irregular in form, 
mere angular masses perhaps. 

The network of orbits of the asteroids is incon- 
ceivable complicated. Nevertheless, there is a wide 
variation in their average distance from the sun, 
and their periods of traveling round him vary in a 
similar manner, the shortest being only about three 

Sei. Vol. 2—9 . 



258 ASTRONOMY TO-DAY 

years. While the longest is nearly nine years in 
duration, the average of all their periods is a little 
over four years. The gap in the zone of asteroids, 
at a distance from the sun equal to about five-eighths 
that of Jupiter^ is due to the excessive disturbing 
action of Jupiter, whose periodic time is just twice 
as long as that of a theoretical planet at this distance. 

The average inclination of their orbits to the 
plane of the ecliptic is not far from 8 degrees. But 
the orbit of Pallas, for example, is inclined 35 de- 
grees, and the eccentricities of the asteroid orbits 
are equally erratic and excessive. Both eccentricity 
and inclination of orbit at times suggest a possible 
relation to cometary orbits, but nothing has ever 
been definitely made out connecting asteroids and 
comets in a related origin. 

No comprehensive theory of the origin of the as- 
teroid group has yet been propounded that has met 
with universal acceptance. According to the nebular 
hypothesis the original gaseous material, which 
should have been so concentrated as to form a planet 
of ordinary type, has in the case of the asteroids col- 
lected into a multitude of small masses instead of 
simply one. That there is a sound physical reason 
for this can hardly be denied. According to the 
Laplacian hypothesis, the nearness of the huge 
planetary mass of Jupiter just beyond their orbits 
produced violent perturbations which caused the 
original ring of gaseous material to collect into 
fragmentary masses instead of one considerable 
planet. The theory of a century ago that an original 
great planet was shattered by internal explosive 
forces is no longer regarded as tenable. 

To astronomers engaged upon investigation of 
distances in the solar system, the asteroid group has 



THE LITTLE PLANETS 259 

proved very useful. The late Sir David Gill em- 
ployed a number of them in a geometrical research 
for finding the sun's distance, and more recently the 
discovery of Eros (433) has made it possible to 
apply a similar method for a like purpose when it 
approaches nearest to the earth in 1924 and 1931. 
Then the distance of Eros will be less than half that 
of Mars or even Venus at their nearest. 

When the total number of asteroids discovered 
has reached 1,000, with accurate determination of 
all their orbits, we shall have sufficient material for 
a statistical investigation of the group which ought 
to elucidate the question of its origin, and bear on 
other problems of the cosmogony yet unsolved. 
Present methods of discovery of the asteroids by 
photography replace entirely the old method by 
visual observation alone, with the result that dis- 
coveries are made with relatively great ease and 
rapidity. 



CHAPTER XXXVI 

THE GIANT PLANET 

I CAN never forget as a young boy my first 
glimpse of the planet Jupiter and his moons; it 
was through a bit of a telescope that I had put 
together with my own hands ; a tube of pasteboard, 
and a pair of old spectacle lenses that chanced to 
be lying about the house. 

In the field of view I saw five objects; four of 
them looking quite alike, and as if they were stars 
merely (they were Jupiter's moons), while the 
fifth was vastly larger and brighter. It was cir- 
cular in shape, and I thought I could see a faint 
darkish line across the middle of it. 

This experience encouraged me immensely, and 
I availed myself eagerly of the first chance to see 
Jupiter through a bigger and better glass. Then 
I saw at once that I had observed nothing wrongly, 
but that I had seen only the merest fraction of 
what there was to see. 

In the first place, the planet's disk was not per- 
fectly circular, but slightly oval. Inquiring into 
the cause of this, we must remember that Jupiter 
is actually not a flat disk but a huge ball or globe, 
more than ten times the diameter of the earth, 
which turns swiftly round on its axis once every 
ten hours as against the earth's turning round in 
twenty-four hours. Then it is easy to see how the 
centrifugal force bulges outward the equatorial 

260 



THE GIANT PLANET 261 

regions of Jupiter, so that the polar regions are 
correspondingly drawn inward, thereby making the 
polar diameter shorter than the equatorial one, 
which is in line with the moons or satellites. The 
difference between the two diameters is very 
marked, as much as one part in fifteen. All the 
planets are slightly flattened in this way, but Ju- 
piter is the most so of all except Saturn. 

The little darkish line across the planet's middle 
region or equator was found to be replaced by sev- 
eral such lines or irregular belts and spots, often 
seen highly colored, especially with reflecting tele- 
scopes; and they are pei'petually changing their 
mutual relation and shapes, because they are not 
solid territory or land on Jupiter, but merely the 
outer shapes of atmospheric strata, blown and torn 
and twisted by atmospheric circulation on this 
planet, quite the same as clouds in the atmosphere 
on the earth are. 

Besides this the axial turning of Jupiter brings 
an entirely different part of the planet into view 
every two or three hours ; so that in making a map 
or chart of the planet, an arbitrary meridian must 
be selected. Even then the process is not an easy 
one, and it is found that spots on Jupiter's equator 
turn round in 9 hours 50 minutes, while other 
regions take a few minutes longer, the nearer the 
poles are approached. The Great Red Spot, about 
30,000 miles long and a quarter as much in breadth 
has been visible for about half a century. Bolton, 
an English observer, has made interesting studies of 
it very recently. 

The four moons, or satellites, which a small 
telescope reveals, are exceedingly interesting on 
many accounts. They were the first heavenly 



262 ASTRONOMY TO-DAY 

bodies seen by the aid of the telescope, Galileo hav- 
ing discovered them in 1610. They travel round 
Jupiter much the same as the moon does round the 
earth, but faster, the innermost moon about four 
times per week, the second moon about twice a 
week, the third or largest moon (larger than the 
planet Mercury) once a week, and the outermost 
in about sixteen days. The innermost is about 
260,000 miles from Jupiter, and the outermost more 
than a million miles. From their nearness to the 
huge and excessively hot globe of Jupiter, some 
astronomers. Proctor especially, have inclined to the 
view that these little bodies may be inhabited. 

Jupiter has other moons ; a very small one, close 
to the planet, which goes round in less than twelve 
hours, discovered by Barnard in 1892. Four others 
are known, very small and faint and remote from 
the planet, which travel slowly round it in orbits of 
great magnitude. The ninth, or outermost, is at a 
distance of fifteen and one-half million miles from 
Jupiter, and requires nearly three years in going 
round the planet. It was discovered by Nicholson 
at the Lick Observatory in 1914. The eighth was 
discovered by Melotte at Greenwich in 1908, and 
is peculiar in the great angle of 28 degrees, at 
which its orbit is inclined to the equator of Jupiter. 
The sixth and seventh satellites revolve round Ju- 
piter inside the eighth satellite, but outside the 
orbit of IV; and they were discovered by photog- 
raphy at the Lick Observatory in 1905 by Perrine, 
now director of the Argentine National Observ- 
atory at Cordoba. 

The ever-changing positions of the Medicean 
moons, as Galileo called the four satellites that he 
discovered — ^their passing into the shadow in 



THE GIANT PLANET 263 

eclipse, their transit in front of the disk, and their 
occultation behind it — form a succession of phe- 
nomena which the telescopist always views with 
delight. The times when all these events take 
place are predicted in the ''Nautical Almanac,'' many 
thousand of them each year, and the predictions 
cover two or three years in advance. 

Jupiter, as the naked eye sees him high up in 
the midnight sky, is the brightest of all the planets 
except Venus ; indeed, he is five times brighter than 
Sirius, the brightest of all the fixed stars. His 
stately motion among the stars will usually be 
visible by close observation from day to day, and 
his distance from the earth, at times when he is 
best seen, is usually about 400 million miles. 
Jupiter travels all the way round the sun in twelve 
years; his motion in orbit is about eight miles 
a second. 

The eclipses of Jupiter's moons, caused by pass- 
ing into the shadow of the planet, would take place 
at almost perfectly regular intervals, if our dis- 
tance from Jupiter were invariable. But it was 
early found out that while the earth is approach- 
ing Jupiter the eclipses take place earlier and 
earlier, but later and later when the earth is mov- 
ing away. The acceleration of the earliest eclipse 
added to the retardation of the latest makes 1,000 
seconds, which is the time that light takes in cross- 
ing a diameter of the earth's orbit round the sun. 
Now the velocity of light is well known to be 186,- 
300 miles per second, so we calculate at once and 
very simply that the sun's distance from the earth, 
which is half the diameter of the orbit, equals 500 
times 186,300, or 93,000,000 miles. 



CHAPTER XXXVII 

THE RINGED PLANET 

SATURN is the most remote of all the planets 
that the ancient peoples knew ans^thing about. 
These anciently known planets are sometimes 
called the lucid or naked-eye planets — ^ve in num- 
ber: Mercury, Venus, Mars, Jupiter, and Saturn. 
Saturn shines as a first-magnitude star, with a 
steady straw-colored light, and is at a distance of 
about 800 million miles from the earth when best 
seen. Saturn travels completely round the sun in 
a little short of thirty years, and the telescope, 
when turned to Saturn, reveals a unique and 
astonishing object; a vast globe somewhat similar 
to Jupiter, but surrounded by a system of rings 
wholly unlike anything else in the universe, as far 
as at present known; the whole encircled by a 
family of nine moons or satellites. The Saturnian 
system, therefore, is regarded by many as the most 
wonderful and most interesting of all the objects 
that the telescope reveals. 

At first the flattening of the disk of Saturn is 
not easily made out, but every fifteen years (as 
1921 and 1936) the earth comes into a position 
where we look directly at the thin edge of the 
rings, causing them to completely disappear. Then 
the remarkable flattening of the poles of Saturn is 
strikingly visible, amounting to as much as one- 
tenth of the entire diameter. The atmospheric belt 
system is also best seen at these times. 

264 



THE RINGED PLANET 265 

But the rings of Saturn are easily the most fas- 
cinating features of the system. They can never 
be seen as if we were directly above or beneath 
the planet so they never appear circular, as they 
really are in space, but always oval or elliptical in 
shape. The minor axis or greatest breadth is 
about one-half the major axis or length. The lat- 
ter is the outer ring's actual diameter, and it 
amounts to 170,000 miles, or two and one-half times 
the diameter of Saturn's globe. 

There are in fact no less than four rings; an 
outer ring, sometimes seen to be divided near its 
^ middle; an inner, broader and brighter ring; and 
an innermost dusky, or crape ring, as it is often 
called. This comes within about 10,000 miles of 
the planet itself. After the form and size of the 
rings were well made out, their thickness, or rather 
lack of thickness, was a great puzzle. 

If a model about a foot in diameter were cut out 
of tissue paper, the relative proportion of size and 
thickness would be about right. In space the thick- 
ness is very nearly 100 miles, so that, when we look 
at the ring system edge-on, it becomes all but in- 
visible except in very large telescopes. Clearly a 
ring so thin cannot be a continuous solid object 
and recent observations have proved beyond a 
doubt that Saturn's rings are made up of millions 
of separate particles moving round the planet, each 
as if it v/ere an individual satellite. 

Ever since 1857 the true theory of the constitu- 
tion of the Saturnian ring has been recognized on 
. theoretic grounds, because Clerke-Maxwell founded 
the dynamical demonstration that the rings could 
be neither fluid nor solid, so that they must be 
made up of a vast multitude of particles traveling 



266 ASTRONOMY TO-DAY 

round the planet independently. But the physical 
demonstration that absolutely verified this conclu- 
sion did not come until 1895, when, as we have 
said in a preceding chapter, Keeler, by radial veloc- 
ity measures on different regions of the ring by 
means of the spectroscope, proved that the inner 
parts of the ring travel more swiftly round the 
planet than the outer regions do. And he further 
showed that the rates of revolution in different 
parts of the ring exactly correspond to the periods 
of revolution which satellites of Saturn would have, 
if at the same distance from the center of the 
planet. The innermost particles of the dusky ring, 
for example, travel round Saturn in about five 
hours, while the outermost particles of the outer 
bright ring take 137 hours to make their revolu- 
tion. For many years it was thought that the Sa- 
turnian ring system was a new satellite in process 
of formation, but this viev/ is no longer enter- 
tained ; and the system is regarded as a permanent 
feature of the planet, although astronomers are 
not in entire agreement as to the evolutionary 
process by which it came into existence — whether 
by some cosmic cataclysm, or by gradual develop- 
ment throughout indefinite aeons, as the rest of the 
solar system is thought to have come to its present 
state of existence. Possibly the planetesimal hy- 
pothesis of Chamberlin and Moulton affords the true 
explanation, as the result of a rupture due to exces- 
sive tidal strain. 



CHAPTER XXXVIII 

THE FARTHEST PLANETS 

ON the 13th of March, 1781, between 10 and 11 
P. M., as Sir William Herschel was sweeping 
the constellation Gemini with one of his great re- 
flecting telescopes, one star among all that passed 
through the field of view attracted his attention. 
Removing the eyepiece and applying another with 
a highere magnifying power, he found that, unlike 
all the other stars, this one had a small disk and was 
not a mere point of light, as all the fixed stars seem 
to be. 

A few nights' observation showed that the 
stranger was moving among the stars, so he thought 
it must be a comet ; but a week's observation follow- 
ing showed that he had discovered a new member 
of the planetary system, far out beyond Saturn, 
which from time immemorial had been assumed to 
be the outermost planet of all. This, then, was the 
first real discovery of a planet, as the finding of the 
satellites of Jupiter had been the first of all astro- 
nomical discoveries. HerscheFs discovery occasioned 
great excitement, and he named the new planet 
Georgium Sidus or the Georgian, after his King. 
The King created him a knight and gave him a 
pension, besides providing the means for building a 
huge telescope, 40 feet long, with which he subse- 
quently made many other astronomical discoveries. 
The planet that Herschel discovered is now called 
Uranus. 

267 



268 ASTRONOMY TO-DAY 

Uranus is an object not wholly impossible to see 
with the naked eye, if the sky background is clear 
and black, and one knows exactly where to look for 
it. Its brightness is about that of a sixth magnitude 
star or a Httle fainter. Its average distance from 
the sun is about 1,800 million miles and it takes 
eighty-four years to complete its journey round the 
sun, traveling only a little more than four miles a 
second. When we examine Uranus closely with a 
large telescope, we find a small disk slightly greenish 
in tint, very slightly flattened, and at times faint 
bands or belts are apparently seen. Uranus is about 
80,000 miles in diameter, and is probably surrounded 
by a dense atmosphere. Its rotation time is 10 h. 
50 m. 

Uranus is attended by four moons or satellites, 
named Ariel, Umbriel, Titania, and Oberon, the last 
being the most remote from the planet. This system 
of satellites has a remarkable peculiarity : the plane 
of the orbits in which they travel round Uranus is 
inclined about 80 degrees to the plane of the ecliptic, 
so that the satellites travel backward, or in a retro- 
grade direction; or we might regard their motion 
as forward, or direct, if we considered the planes 
of their orbits inclined at 100 degrees. 

For many years after the discovery of Uranus 
it was thought that all the great bodies of the solar 
system had surely been found. Least of all was any 
planet suspected beyond Uranus until the mathe- 
matical tables of the motion of Uranus, although 
built up and revised with the greatest care and thor- 
oughness, began to show that some outside influence 
was disturbing it in accordance with Newton's 
law of gravitation. The attraction of a still more 
distant planet would account for the disturbance, 



THE FARTHEST PLANETS 269 

and since no such planet was visible anywhere a 
mathematical search for it was begun. 

NEPTUNE 

Wholly independently of each other, two young 
astronomers, Adams of England and LeVerrier of 
France, undertook to solve the unique problem of 
finding out the position in the sky where a planet 
might be found that would exactly account for the 
irregular motion of Uranus. Both reached practi- 
cally identical results. Adams was first in point of 
time, and his announcement led to the earliest ob- 
servation, without recognition of the new planet 
(July 30, 1846), although it was LeVerrier's work 
that led directly to the new planet's being first seen 
and recognized as such (September 23, 1846). Fig- 
uring backward, it was found that the planet had 
been accidentally observed in Paris in 1795 , but its 
planetary character had been overlooked. 

Neptune is the name finally assigned to this his- 
torical planet. It is thirty times farther from the 
sun than the earth, or 2,800 million miles; its 
velocity in orbit is a little over three miles per 
second, and it consumes 164 years in going once 
completely round the sun. So faint is it that a 
telescope of large size is necessary to show it plainly. 
The brightness equals that of a star of the eighth 
magnitude, and with a telescope of sufficient mag- 
nifying power, the tiny disk can be seen and meas- 
ured. The planet is about 30,000 miles in diameter, 
and is not known to possess more than one moon or 
satellite. If there are others, they are probably too 
faint to be seen by any telescope at present in 
existence. 



CHAPTER XXXIX 

THE TRANS-NEPTUNIAN PLANET 

INVESTIGATION of the question of a possible 
trans-Neptunian planet was undertaken by the 
writer in 1877. As Neptune requires 164 years to 
travel completely round the sun, and the period 
during which it has been carefully observed em- 
braces only half that interval, clearly its orbit can- 
not be regarded as very well known. Any possible 
deviations from the mathematical orbit could not 
therefore be traced to the action of a possible un- 
known planet outside. But the case was different 
with Uranus, which showed very slight disturb- 
ances, and these were assumed to be due to a pos- 
sible planet exterior to both Uranus and Neptune. 
As a position for this body in the heavens was indi- 
cated by the writer's investigation, that region of 
the sky was searched by him with great care in 
1877-1878 with the twenty-six-inch telescope at 
Washington; and photographs of the same region 
were afterward taken by others, though only with 
negative results. 

In 1880, Forbes of Edinburgh published his in- 
vestigation of the problem from an entirely inde- 
pendent angle. Families of comets have long been 
recognized whose aphelion distances correspond so 
nearly with the distances of the planets that these 
comet families are now recognized as having been 
created by the several planets, which have reduced 

270 



THE TRANS-NEPTUNIAN PLANET 271 

the high original velocities possessed by the comets 
on first entering the solar system. 

Their orbits have ever since been ellipses with 
their aphelia in groups corresponding to the dis- 
tances of the planets concerned. Jupiter has a large 
group of such comets, also Saturn. Uranus and 
Neptune likewise have their families of comets, and 
Forbes found two groups with average distances 
far outside of Neptune ; from which he drew the in- 
ference that there are two trans-Neptunian planets. 
The position he assigned to the inner one agreed 
fairly well with the writer's planet as indicated by 
unexplained deviations of Uranus. 

The theoretical problem of a trans-Neptunian 
planet has since been taken up by Gaillot and Lau 
of Paris, the late Percival Lowell, and W. H. Picker- 
ing of Harvard. The photographic method of search 
will, it is expected, ultimately lead to its discovery. 
On account of the probable faintness of the planet, 
at least the twelfth or thirteenth magnitude, Met- 
calf 's method of search is well adapted to this prac- 
tical problem. When near its opposition the motion 
of Neptune retrograding among the stars amounts 
to five seconds of arc in an hour; while the trans- 
Neptunian planet would move but three seconds. 
By shifting the plate this amount hourly during ex- 
posure, the suspected object would readily be de- 
tected on the photographic plate as a minute and 
nearly circular disk, all the adjacent stars being 
represented by short trails. 

Interest in a possible planet or planets outside the 
orbit of Neptune is likely to increase rather than 
diminish. To the ancients seven was the perfect 
number, there were seven heavenly bodies already 
known, so there could be no use whatever in looking 



272 ASTRONOMY TO-DAY 

for an eighth. The discovery of Uranus in 178^ 
proved the futility of such logic, and Neptune fol- 
lowed in 1846 with further demonstration, if need 
be. The cosmogony of the present day sets no outer 
limit to the solar system, and some astronomers ad- 
vocate the existence of many trans-Neptunian 
planets. 



CHAPTER XL 

COMETS—THE HAIRY STARS 

COMETS — ^hairy stars, as the origin of the name 
would indicate — are the freaks of the heavens. 
Of great variety in shape, some with heads and some 
without, some with tails and some without, moving 
very slowly at one time and with exceedingly high 
velocity at another, in orbits at all possible angles 
of inclination to the general plane of the planetary 
paths round the sun, their antics and irregularities 
were the wonder and terror of the ancient world, 
and they are keenly dreaded by superstitious people 
even to the present day. 

Down through the Middle Ages the advent of a 
comet was regarded as: 

Threatening the world with famine, plague and war; 

To princes, death ; to kingdoms, many curses ; 

To all estates, inevitable losses; 

To herdsmen, rot; to plowmen, hapless seasons; 

To sailors, storms; to cities, civil treasons. 

Comets appeared to be marvelous objects, as well 
as sinister, chiefly because they bid apparent de- 
fiance to all law. Kepler had shown that the moon 
and the planets travel in regular paths — slightly 
elliptical to be sure, but nevertheless unvarying. 
None of the comets were known to follow regular 
paths till the time of Halley late in the seventeenth 
century, when, as we have before told, a fine comet 

273 



274 ASTRONOMY TO-DAY 

made its appearance, and Halley calculated its orbit 
with much precision. Comparing this with the 
orbits of comets that had previously been seen, he 
found its path about the sun practically identical 
with that of at least two comets previously observed 
in 1531 and 1607. 

So Halley ventured to think all these comets were 
one and the same body, and that it traveled round the 
sun in a long ellipse in a period of about seventy-five 
or seventy-six years. We have seen how his pre- 
diction of its return in 1758 was verified in every 
particular. On the comet's return in 1910, Crowell 
and Crommelin of Greenwich made a thorough 
mathematical investigation of the orbit, indicating 
that the year 1986 will witness its next return to the 
sun. 

There is a class of astronomers known as comet- 
hunters, and they pass hours upon hours of clear, 
sparkling, moonless nights in search for comets. 
They are equipped with a peculiar sort of telescope 
called a comet-seeker, which has an object glass 
usually about four or five inches in diameter, and a 
relatively short length of focus, so that a larger 
field of view may be included. Regions near the 
poles of the heavens are perhaps the most fruitful 
fields for search, and thence toward the sun till its 
light renders the sky too bright for the finding of 
such a faint object as a new comet usually is at the 
time of discovery. Generally when first seen it 
resembles a small circular patch of faint luminous 
cloud. 

When a suspect is found, the first thing to do is 
to observe its position accurately with relation to the 
surrounding stars. Then, if on the next occasion 
when it is seen the object has moved, the chances 



COMETS^THE HAIRY STARS 275 

are that it is a comet; and a few days' observation 
will provide material from which the path of the 
comet in space can be calculated. By comparing this 
with the complete lists of comets, no¥/ about 700 in 
number, it is possible to tell whether the comet is 
a new one, or an old one returning. The total number 
of comets in the heavens must be very great, and 
thousands are doubtless passing continually unde^ 
tected, because their light is v/holly overpowered 
by that of the sun. Of those that are known, per- 
haps one in twelve develops into a naked-eye comet, 
and in some years six or seven will be discovered. 
With sufficiently powerful telescopes, there are as 
a rule not many weeks in the year when no comet 
is visible. Brilliant naked-eye comets are, however, 
infrequent. 

Comets, except Halley's, generally bear the name 
of their discoverer, as Donati (1858), and Pons- 
Brooks (1893) . Pons was a very active discoverer of 
comets in France early in the nineteenth century : he 
was a doorkeeper at the observatory of Marseilles, 
and his name is now more famous in astronomy than 
that of Thulis, then the director of the Observatory, 
who taught and encouraged him. Messier was 
another very successful discoverer of comets ii| 
France, and in America we have had many : Swift, 
Brooks, and Barnard the most successful. 

How bright a comet will be and how long it will 
be visible depends upon many conditions. So the 
comets vary much in these respects. The first comet 
of 1811 was under observation for nearly a year and 
a half, the longest on record till Halley's in 1910. 
In case a comet eludes discovery and observation 
until it has passed its perihelion, or nearest point to 
the sun, its period of visibility may be reduced to a 



276 ASTRONOMY TO-DAY 

few weeks only. The brightest comets on record 
were visible in 1843 and 1882 : so brilliant were they 
that even the effulgence of full daylight did not over- 
power them. In particular the comet of 1843 was 
not only excessively bright, but at its nearest ap- 
proach to the earth its tail swept all the way across 
the sky from one horizon to the other. It must have 
looked very much like the straight beam of an 
enormous searchlight, though very much brighter. 

The tails of comets are to the naked eye the most 
compelling thing about them, and to the ancient 
peoples they were naturally most terrifying. Their 
tails are not only curved, but sometimes curved with 
varying degrees of curvature, and this circumstance 
adds to their weirdness of appearance. If we ex- 
amine the tail of a comet with a telescope, it vanishes 
as if there were nothing to it: as indeed one may 
almost say there is not. Ordinarily, only the head 
of the comet is of interest in the telescope. When 
first seen there is usually nothing but the head 
visible, and that is made up of portions which 
develop more or less rapidly, presenting a suc- 
cession of phenomena quite different in different 
comets. 

When first discovered a comet is usually at a great 
distance from the sun, about the distance of Jupiter ; 
and we see it, not as we do the planets, by sunlight 
reflected from them, but by the comet's own light. 
This is at that time very faint, and nearly all comets 
at such a distance look alike: small roundish hazy 
patches of faint, cloudlike light, with very often a 
concentration toward the center called the nucleus, 
on the average about 4,000 miles in diameter. Ap- 
proach toward the sun brightens up the comet more 
and more, and the nucleus usually becomes very 



COMETS— THE HAIRY STARS 277 

much brighter and more starlike. Then on the 
sunward side of the nucleus, jetlike streamers or 
envelopes appear to be thrown off, often as if in 
parallel curved strata, or concentrically. As they 
expand and move outward from the nucleus, these 
envelopes grow fainter and are finally merged iri 
the general nebulosity known as the comet's head, 
which is anywhere from 30,000 to 100,000 miles in 
diameter. As a rule, this is an orderly development 
which can be watched in the telescope from hour to 
hour and from night to night; but occasionally a 
cometary visitor is quite a law to itself in develop- 
ment, presenting a fascinating succession of unpre- 
dictable surprises. 

Then follows the development of the comet's tail, 
perhaps more striking than anything that has pre- 
ceded it. Here a genuine repulsion from the sun 
appears to come into play. It may be an electrical 
repulsion. Much of the material projected from the 
comet's nucleus, seems to be driven backward or re- 
pelled by the sun, and it is this that goes to form 
the tail. The particles which form the tail then 
travel in modified paths which nevertheless can be 
calculated. The tail is made up of these luminous 
particles and it expands in space much in the form 
of a hollow, horn-shaped cone, the nucleus being 
near the tip of the horn. 

Some comets possess multiple tails with different 
degrees of curvature, Donati's for example. Usually 
there is a nearly straight central dark space, mark- 
ing the axis of the comet, and following the nucleus. 
But occasionally this is replaced by a thin light 
streak very much less in breadth than the diameter 
' of the head. Cometary tails are sometimes 100 mil- 
lion miles in length. 



278 ASTRONOMY TO-DAY 

Three diiferent types of cometary tails are recog- 
nized. First, the long straight ones, apparently made 
up of matter repelled by the sun twelve to fifteen 
times more powerfully than gravitation attracts it. 
Such particles must be brushed away from the 
comet's head with a velocity of perhaps five miles 
a second, and their speed is continually increasing. 
Probably these straight tails are due to hydrogen. 
The second type tails are somev/hat curved, or plume- 
like, and they form the most common type of comet- 
ary tail. In them the sun's repulsion is perhaps 
twice its gravitational attraction, and hydrocarbons 
in some form appear to be responsible for tails of 
this character. Then there is a third type, much less 
often seen, short and quickly curving, probably due 
to heavier vapors, as of chlorine, or iron, or sodium, 
in which the repulsive force is only a small fraction 
of that of gravitation. 

Many features of this theory of cometary tails 
are borne out by examination of their light with the 
spectroscope, although the investigation is as yet 
fragmentary. It is evident that the tail of a comet 
is formed at the expense of the substance of the 
nucleus and head; so that the matter repelled is 
forever dissipated through the regions of space 
which the comet has traveled. Comets must lose 
much of their original substance every time they 
return to perihelion. Comets actually age, there- 
fore, and grow less and less in magnitude of material 
as well as brightness, until they are at last opaque, 
nonluminous bodies which it becomes impossible to 
follow with the telescope. 



CHAPTER XLI 

P^HERE DO COMETS COME FROM? 

WHERE do comets come from? The answer to 
this question is not yet fully made out. Most 
likely they have not all had a similar origin, and 
theories are abundant. Apparently they come into 
the solar system from outer space, from any direc- 
tion whatsoever. The depths of interstellar space 
seem to be responsible for most, if not all, of the new 
ones. Whether they have come from other stars or 
stellar systems we cannot say. 

While comets are tremendous in size or volume, 
their mass or the amount of real substance in them 
is relatively very slight. We know this by the effect 
they produce on planets that they pass near, or 
rather by the effect that they fail to produce. 
The earth's atmosphere weighs about one two hun- 
dred and fifty thousandth as much as the earth it- 
self, but a comet's entire mass must be vastly less 
than this. Even if a comet were to collide with the 
earth head on, there is little reason to believe that 
dire catastrophe would ensue. At least twice the 
earth is known to have passed through the tail of 
a comet, and the only effect noticed was upon the 
comet itself; its orbit had been modified somewhat 
by the attraction of the earth. If the comet were 
a small one, collision with any of the planets would 
result in absorption and dissipation of the comet 
into vapor. 

279 



280 ASTRONOMY TO-DAY 

The whole of a large comet has perhaps as much 
mass or weight as a sphere of iron a hundred 
miles in diameter. Even this could not wreck the 
earth, but the effect would depend upon what part 
of the earth was hit. A comet is very thin and 
tenuous, because its relatively small mass is dis- 
tributed through a volume so enormous. So it is 
probable that the earth's atmosphere could scatter 
and burn up the invading comet, and we should 
have only a shower of meteors on an unprecedented 
scale. Diffusion of noxious gases through the at- 
mosphere might vitiate it to some extent, though 
probably not enough to cause the extinction of 
animal life. 

Every comet has an interesting history of its 
own, almost indeed unique. One of the smallest 
comets and the briefest in its period round the sun 
is known as Encke's comet. It is a telescopic comet 
with a very short tail, its time of revolution is 
about three and a half years, and it exhibits a 
remarkable contraction of volume on approach to 
the sun. 

Biela's comet has a period about twice as long. 
At one time it passed within about 15 million miles 
of the earth, and somewhere about the year 1840 
this comet divided into two distinct comets, which 
traveled for months side by side, but later sepa- 
rated and both have since completely disappeared. 
Perhaps the most beautiful of all comets is that dis- 
covered by Donati of Florence in 1858. Its coma 
presented the development of jets and envelopes in 
remarkable perfection, and its tail was of the sec- 
ondary or hydrocarbon type, but accompanied by 
two faint streamer tails, nearly tangential to the 
main tail and of the hydrogen type, Donati's 



WHERE DO COMETS COME FROM? 281 

comet moves in an ellipse of extraordinary length, 
and it will not return to the sun for nearly 2,000 
years. 

The most brilliant comet of the last half century 
is known as the great comet of 1882. In a clear 
sky it could readily be seen at midday. On Septem- 
ber 17 it passed across the disk of the sun and was 
practically as bright as the surface of the sun itself. 
The comet had a multiple nucleus and a hydro- 
carbon tail of the second type, nearly a hundred 
million miles in length. Doubtless this great comet 
is a member of what is known as a cometary group, 
which consists of comets having the same orbit 
and traveling tandem round the sun. The comets 
of 1668, 1843, 1880, 1882 and 1887 belong to this 
particular group, and they all pass within 300,000 
miles of the sun's surface, at a maximum velocity 
exceeding 300 miles a second. They must there- 
fore invade the regions of the solar corona, the 
inference being that the corona as well as the comet 
is composed of exceedingly rare matter. 

Photography of comets has developed remark- 
ably within recent years, especially under the deft 
manipulation of Barnard, whose plates, in par- 
ticular during his residence at the Lick Observ- 
atory on Mount Hamilton, California, show the 
features of cometary heads and tails in excellent 
definition. Halley's comet, at the 1910 appari- 
tion, was particularly well photographed at many 
observatories. 

The question is often asked, When will the next 
comet come? If a large bright comet is meant, 
astronomers cannot tell. At almost any time one 
may blaze into prominence within only a few days. 
During the latter half of the last century, bright 



282 ASTRONOMY TQ-DAY 

comets appeared at perihelion at intervals of eight 
years on the average. Several of the lesser and 
fainter periodic comets return nearly every year, 
but they are mostly telescopic, and are rarely seen 
except by astronomers who are particularly inter- 
ested in observing them. 



CHAPTER XLII 

METEORS AND SHOOTING STARS 

"PIALLING STARS," or "shooting stars," have 
A been familiar sights in all ages of the world, but 
the ancient philosophers thought them scarcely- 
worthy of notice. According to Aristotle they were 
mere nothings of the upper atmosphere, of no more 
account than the general happenings of the weather. 
But about the end of the eighteenth century and 
the beginning of the nineteenth the insufficiency of 
this view began to be fully recognized, and inter- 
planetary space was conceived as tenanted by shoals 
of moving bodies exceedingly small in mass and 
dimension as compared with the planets. 

Millions of these bodies are all the time in col- 
lision with the outlying regions of our atmosphere; 
and by their impact upon it and their friction in 
passing swiftly through it, they become heated to 
incandescence, thus creating the luminous appear- 
ances commonly known as shooting stars. For the 
most part they are consumed or dissipated in vapor 
before reaching the solid surface of the earth; but 
occasionally a luminous cloud or streak is left glow- 
ing in the wake of a large meteor, which sometimes 
remains visible for half an hour after the passage 
of the meteor itself. These mistlike clouds pro- 
jected upon the dark sky have been especially studied 
by Trowbridge of Columbia University. 

Many more meteors are seen during the morning 
hours, say from four to six, than at any other nightly 

283 



284 ASTRONOMY TO-DAY 

period of equal length, because the visible sky is at 
that time nearly centered around the general di- 
rection toward which the earth is moving in its 
orbit round the sun ; so that the number of meteors 
that would fall upon the earth if at rest is increased 
by those which the earth overtakes by its own 
motion. Also from January to July while the earth 
is traveling from perihelion to aphelion, fewer 
meteors are seen than in the last half of the year; 
but this is chiefly because of the rich showers en- 
countered in August and November. 

Although the descent of meteoric bodies from the 
sky was pretty generally discredited until early in 
the nineteenth century, such falls had nevertheless 
been recorded from very early times. They were 
usually regarded as prodigies or miracles, and such 
stones were commonly objects of worship among 
ancient peoples. For example, the Phrygian Stone, 
known as the "Diana of the Ephesians which fell 
down from Jupiter," was a famous stone built into 
the Kaaba at Mecca, and even to-day it is revered by 
Mohammedans as a holy relic. Perhaps the earliest 
known meteoric fall is that historically recorded in 
the Parian Chronicle as having occurred in the 
island of Crete, B. C. 1478. Also in the imperial 
museum of Petrograd is the Pallas or Krasnoiarsk 
iron, perhaps three-quarters of a ton in weight, 
found in 1772 by Pallas, the famous traveler, at 
Krasnoiarsk, Siberia. 

But a fall of meteoric stones that chanced upon 
the department of Orne, France, in 1805, led to a 
critical investigation by Biot, the distinguished 
physicist and academician. According to his report 
a violent explosion in the neighborhood of L'Aigle 
had been heard for a distance of seventy-five miles 



METEORS AND SHOOTING STARS 285 

around, and lasting five or six minutes, about 1 P.M. 
on Tuesday, April 26. From several adjoining 
towns a rapidly moving fireball had been seen in a 
sky generally clear, and there was absolutely no 
room for doubt that on the same day many stones 
fell in the neighborhood of L'Aigle. Biot estimated 
their number between two and three thousand, and 
they were scattered over an elliptical area more 
than six miles long, and two and a half miles broad. 
Thenceforward the descent of meteoric matter from 
outer space upon the earth has been recognized as 
an unquestioned fact. 

The origin of these bodies being cosmic, meteors 
may be expected to fall upon the earth without ref- 
erence to latitude, or season, or day and night, or 
weather. On entering our upper atmosphere their 
temperature must be that of space, many hundred 
degrees below zero ; and their velocities range from 
ten miles per second upward. But atmospheric re- 
sistance to their flight is so great that their velocity 
is quickly reduced : at ground impact it does not ex- 
ceed a few hundred feet per second. On January 1, 
1869, several meteoric stones fell on ice only a few 
inches thick in Sweden, rebounding without either 
breaking through the ice or being themselves 
fractured. 

Naturally the flight of a meteor through the at- 
mosphere will be only a few seconds in duration, 
and owing to the sudden reduction of velocity, it will 
continue to be luminuous throughout only the upper 
part of its course. Visibility generally begins at an 
elevation of about seventy miles, and ends at 
perhaps half that altitude. 

What is the origin of meteors? Theories there 
are in great abundance: that they come from the 



286 ASTRONOMY TO-DAY 

sun, that they come from the moon, that they come 
from the earth in past ages as a result of volcanic 
action, and so on. But there are many difficulties in 
the way of acceptance of these and several other 
theories. That all meteors were originally parts of 
cometary masses is however a theory that may be 
accepted without much hesitation. 

Comets have been known to disintegrate. Biela's 
comet even disappeared entirely, so that during a 
shower of Biela meteors in November, 1885, an 
actual fragment of the lost comet fell upon the earth, 
at Mazapil, Mexico. And as the Bielid meteors en- 
counter the earth with the relatively low velocity of 
ten miles a second, we may expect to capture other 
fragments in the future. Numerous observers saw 
the weird disintegration of the nucleus of the great 
comet of 1882, well recognized as a member of the 
family of the comet of 1843. As these comets are 
fellow voyagers through space along the same orbit, 
probably all five members of the family, with per- 
haps others, were originally a single comet of un- 
paralleled magnitude. 

The Brooks comet of 1890 affords another instance 
of fragmentary nucleus. The oft-repeated action of 
solar forces tending to disrupt the mass of a comet 
more and more, and scatter its material throughout 
space, the secular dismemberment of all comets be- 
comes an obvious conclusion. During the hundreds 
of millions of years that these forces are known to 
have been operant, the original comets have been 
broken up in great numbers, so that elliptical rings 
of opaque meteoric bodies now travel round the sun 
in place of the comets. 

These bodies in vast numbers are everywhere 
through space^ each too small to reflect an appreci- 



METEORS AND SHOOTING STARS 287 

able amount of sunlight, and becoming visible only 
when they come into collision with our outer at- 
mosphere. The practical identity of several such 
meteor streams and cometary orbits has already 
been established, and there is every reason for as- 
signing a similar origin to all meteoric bodies. 
Meteors, then, were originally parts of comets, which 
have trailed themselves out to such extent that 
particles of the primal masses are liable to be picked 
up anywhere along the original cometary paths. The 
historic records of all countries contain trustworthy 
accounts of meteoric showers. Making due allow- 
ances for the flowery imagery of the oriental, it 
is evident that all have at one time or another seen 
much the same thing. In A. D. 472, for instance, the 
Constantinople sky was reported alive with flying 
stars. In October, 1202, *'stars appeared like waves 
upon the sky; and they flew about like grasshop- 
pers." During the reign of King William II occurred 
a very remarkable shower in which "stars seemed 
to fall like rain from heaven." 

But the showers of November, 1799 and 1833, are 
easily the most striking of all. The sky was filled 
with innumerable fiery trails and there was not a 
space in the heavens a few times the size of the moon 
that was not ablaze with celestial fireworks. Fre- 
quently huge meteors blended their dazzling bril- 
liancy with the long and seemingly phosphorescent 
trails of the shooting stars. 

The interval of thirty-four years between 1799 
and 1833 appeared to indicate the possibility of a 
return of the shower in November of 1866 or 1867, 
and all the people of that day were aroused on this 
subject and made every preparation to witness the 
spectacle. Extemporized observatories were estab- 



288 ASTRONOMY TO-DAY 

lished, watchmen were everywhere on the lookout, 
and bells were to be rung the minute the shower 
began. The newspapers of the day did little to allay 
the fears of the multitude, but the critical days of 
November, 1866, passed with disappointment in 
America. In Europe, however, a fine shower was 
seen, though it was not equal to that of 1833. The 
astronomers at Greenwich counted many thousand 
meteors. In November of 1867, however, American 
astronomers were gratified by a grand display, 
which, although failing to match the general expec- 
tation, nevertheless was a most striking spectacle, 
and the careful preparation for observing it afforded 
data of observation which were of the greatest scien- 
tific value. The actual orbits of these bodies in space 
became known with great exactitude, and it was 
found that their general path was identical with that 
of the first comet of 1866, which travels outward 
somewhat beyond the planet Uranus. When the 
visible paths of these meteors are traced backward, 
all appear as if they originated from the constella- 
tion Leo. So they are known as Leonids, and a re- 
turn of the shower was confidently predicted for 
November, 1900-1901, which for unknown reasons 
failed to appear. 

During the last half century meteors have been 
pretty systematically observed, especially by the as- 
tronomers of Italy and Denning of England, so that 
several hundred distinct showers are now known, 
their radiant points fall in every part of the heavens, 
and there is scarcely a clear moonless night when 
careful watching for meteors will be unrewarded. 
Besides November, the months of August (Per- 
seids), April (Lyrids), and December (Geminids) 
are favorable. Following in tabular form is a fairly 




Two Views of Halley's Comet. Taken with the same camera from 

the same position, one on May 12, and the other on May 15, 1910. 

(Photo, Mt. Wllso)i Solar Observatory.) 




Swift's Comet of 1892. This comet showed extraordinary and rapid 

transformations, one day having a dozen streamers in its tail, another 

only two. {Photo by Prof. E. E. Barnard.) 



METEORS AND SHOOTING STARS 289 

comprehensive list of the meteoric showers of the 
year, with the positions of the radiant points and 
the epochs of the showers according to Denning : 



RADIANT POINT 



Name of Shower 



R. A. 



Decl. Date of Shower 



Quadrantids 

Zeta Cepheids 

Alpha Leonids 

Tau Leonids 

Beta Ursids 

Lyrids 

Gamma Aquarids 

Zeta Herculids 

Eta Pegasids. 

Theta Bootids 

Alpha Capricornids . . . . 

Delta Aquarids 

Perseids 

Omicron Draconids .... 

Zeta Draconids 

Piscids 

Alpha Andromedids 

Epsilon Arietids 

Orionids 

Epsilon Perseids 

Leonids 

Epsilon Taurids 

Andromedids 

Beta Geminids 

Geminids ._. . 

Alpha Ursae Majorids. 
Kappa Draconids 



230° 

331° 

155° 

166° 

161° 

271° 

338° 

246° 

330° 

213° 

304° 

339° 

45° 

291° 

262° 

348° 

4° 

40° 

92° 

61° 

150° 

64° 

25° 

119° 

108° 

161° 

194° 



+ 53° 
+ 56° 
+ 14° 

+ 4° 
+ 58° 
+ 33° 

—2° 
+ 29° 
+ 28° 
+ 53° 
—12° 
—11° 
+ 57° 
+ 60° 
+ 63° 

+ 2° 
+ 28° 
+ 20° 
+ 15° 
+ 35° 
+ 23° 
+ 22° 
+ 43° 
+ 31° 
+ 83° 
+ 58° 
+ 68° 



Jan. 2-4 
Jan. 25 

Feb. 19-March 1 
March 1-4 
March 13-24 
April 20-22 
May 1-6 
May 18-26 
May 30-Jiine 4 
June 27-28 
July 15-28 
July 25-30 
Aug. 10-12 
Aug. 15-25 
Aug. 21-Sept. 2 
Sept. 4-14 
Sept. 27 
Oct. 11-24 
Oct. 17-24 
Nov. 5 
Nov. 13-15 
Nov. 14-25 
Nov. 17-23 
Dec. 1-12 
Dec. 1-14 
Dec. 18-21 
Dec. 18-28 



The year 1916 was exceptional in providing an 
abundant and previously unknown shower on 
June 28, and its stream has nearly the same orbit 
as that of the Pons-Winnecke periodic comet. Use- 
ful observations of meteors are not difficult to make, 
and they are of service to professional astronomers 
investigating the orbits of these bodies, among whom 
are Mitchell and Olivier of the University of Virginia. 

Sci. Vol. 2—10 



CHAPTER XLIII 

METEORITES 

METEORITES, the name for meteors which have 
actually gone all the way through our atmos- 
phere, are never regular in form or spherical. As a 
rule the iron meteorites are covered with pittings or 
thumb marks, due probably to the resistance and im- 
pact of the little columns of air which impede its 
progress, together with the unequal condition and 
fusibility of their surface material. The work done 
by the atmosphere in suddenly checking the meteor's 
velocity appears in considerable part as heat, fusing 
the exterior to incandescence. This thin liquid shell 
is quickly brushed off, making oftentimes a luminous 
train. 

But notwithstanding the exceedingly high temper- 
ature of the exterior, enforced upon it for the brief 
time of transit through the atmosphere, it is prob- 
able that all large meteorites, if they could be reached 
at once on striking the earth, would be found to 
be cold, because the smooth, black, varnishlike 
crust which always incases them as a result of 
intense heat is never thick. On one occasion a 
meteor which was seen to fall in India was dug out 
of the ground as quickly as possible, and found to 
be, not hot as was expected, but coated thickly over 
with ice frozen on it from the moisture in the sur- 
rounding soil. 

290 



METEORITES 291 

As to the composition of shooting stars, and their 
probable mass, and its effect upon the earth, our 
data are quite insufficient. The lines of sodium and 
magnesium have been hurriedly caught in the spec- 
troscope, and, estimating on the basis of the light 
emitted by them, the largest meteors must weigh 
ounces rather than pounds. Nevertheless, it is in- 
teresting to inquire what addition the continual fall 
of many millions daily upon the earth makes to its 
weight; somewhere between thirty and fifty thou- 
sand tons annually is perhaps a conservative esti- 
mate, but even this would not accumulate a layer 
one inch in thickness over the entire surface of the 
earth in less than a thousand million years. 

Many hundreds of the meteors actually seen to 
fall, together with those picked up accidentally, are 
recovered and prized as specimens of great value in 
our collections, the richest of which are now in New 
York, Paris, and London. The detailed investiga- 
tion of them is rather the province of the chemist, 
the crystallographer and the mineralogist than of 
the astronomer whose interest is more keen in their 
life history before they reach the earth. To dis- 
tinguish a stony meteorite from terrestrial rock sub- 
stances is not always easy, but there is usually little 
difficulty in pronouncing upon an iron meteorite. 
These are most frequently found in deserts, because 
the dryness of the climate renders their oxidation 
and gradual disappearance very slow. 

The surface of a suspected iron meteorite is 
polished to a high luster and nitric acid is poured 
upon it. If it quickly becomes etched with a char- 
acteristic series of lines, or a sort of cross-hatching, 
it is almost certain to be a meteorite. Occasionally 
carbon has been found in meteorites, and the ex- 



292 ASTRONOMY TO-DAY 

istence of diamond has been suspected. The miner- 
als composing meteorites are not unlike terrestrial 
materials of volcanic origin, though many of them 
are peculiar to meteorites only. More than one- 
third of all the known chemical elements have been 
found by analysis in meteorites, but not any new 
ones. 

Meteoric iron is a rich alloy containing about ten 
per cent of nickel, also cobalt, tin, and copper in much 
smaller amount. Calcium, chlorine, sodium, and 
sulphur likewise are found in meteoric irons. At 
very high temperatures iron will absorb gases and 
retain them until again heated to red heat. Car- 
bonic oxide, helium, hydrogen, and nitrogen are 
thus imprisoned, or occluded, in meteoric irons in 
very small quantities ; and in 1867, during a London 
lecture by Graham, a room in the Royal Institution 
was for a brief space illuminated by gas brought to 
earth in a meteorite from interplanetary space. 
Meteorites, too, have been most critically investi- 
gated by the biologist, but no trace of germs of 
organic life of any type has so far been found. 
Farrington of Chicago has published a full 
descriptive catalogue of all the North American 
meteorites. 

Recent investigations of the radioactivity of me- 
teorites show that the average stone meteorite is 
much less radioactive than the average rock, and 
probably less than one-fourth as radioactive as 
in average granite. The metallic meteorites ex- 
amined were found about wholly free from radio- 
activity. 

From shooting stars, perhaps the chips of the 
celestial workshop, or more possibly related to the 
planetesimals which the processes of growth of the 



METEORITES 293 

universe have swept up into the vastly greater 
bodies of the universe, transition is natural to the 
stars themselves, the most numerous of the heavenly 
bodies, all shining by their own light, and all in- 
conceivably remote from the solar system, which 
nevertheless appears to be not far removed from 
the center of the stellar universe. 



CHAPTER XLIV 

THE UNIVERSE OF STARS 

OUR consideration of the solar system hitherto 
has kept us quite at home in the universe. The 
outer known planets, Uranus and Neptune, are in- 
deed far removed from the sun, and a few of the 
comets that belong to our family travel to even 
greater distances before they begin to retrace their 
steps sunward. When we come to consider the vast 
majority of the glistening points on the celestial 
sphere — all in fact except the five great planets, 
Mercury, Venus, Mars, Jupiter, and Saturn — ^we are 
dealing with bodies that are self-luminous like the 
sun, but that vary in size quite as the bodies of the 
solar system do, some stars being smaller than the 
sun and others many hundred fold larger than 
he is; some being "giants," and others "dwarfs." 
But the overwhelming remoteness of all these 
bodies arrests our attention and even taxes our 
credulity regarding the methods that astronomers 
have depended on to ascertain their distances 
from us. 

Their seeming countlessness, too, is as bewilder- 
ing as are the distances ; though, if we make actual 
counts of those visible to the naked eye within a 
certain area, in the body of the "Great Bear," for 
example, the great surprise will be that there are so 
few. And if the entire dome of the sky is counted, 
at any one time, a clear, moonless sky would reveal 

294 



THE UNIVERSE OF STARS 295 

perhaps 2,500, so that in the entire sky, northern 
and southern, we might expect to find 5,000 to 6,000 
lucid stars, or stars visible to the naked eye. 

But when the telescope is applied, every accession 
of power increases the myriads of fainter and 
fainter stars, until the number within optical reach 
of present instruments is somewhere between 400 
and 500 millions. But if we were to push the 100- 
inch reflector on Mount Wilson to its limit by pho- 
tography with plates of the highest sensitiveness, 
millions upon millions of excessively faint stars 
would be plainly visible on the plates which the 
human eye can never hope to see directly with any 
telescope present or future, and which would doubt- 
less swell the total number of stars to a thousand 
millions. Recent counts of stars by Chapman and 
Melotte of Greenwich tend to substantiate this 
estimate. 

What have astronomers done to classify or cata- 
logue this vast array of bodies in the sky? Even 
before making any attempt to estimate their num- 
ber, there is a system of classification simply by the 
amount of light they send us, or by their apparent 
stellar magnitudes — not their actual magnitudes, for 
of those we know as yet very little. We speak of 
stars of the "first magnitude,'' of which there are 
about 20, Sirius being the brightest and Regulus Ihe 
faintest. Then there are about 65 of the second, or 
next fainter, magnitude, stars like Polaris, for ex- 
ample, which give an amount of light two and a half 
times less than the average first magnitude star. 
Stars of the third magnitude are fainter than those 
of the second in the same ratio, but their number in- 
creases to 200 ; fourth magnitude, 500 ; fifth magni- 
tude, 1,400 ; sixth magnitude, 5,000, and these are so 



296 ASTRONOMY TO-DAY 

faint that they are just visible on the best nights 
without telescopic aid. 

Decimals express all intermediate graduations of 
magnitude. Astronomers carry the telescopic mag- 
nitudes much farther, till a magnitude beyond the 
twentieth is reached, preserving in every case the 
ratio of two and one-half for each magnitude in re- 
lation to that numerically next to it. Even Jupiter 
and Venus, and the sun and moon, are sometimes 
calculated on this scale of stellar magnitude, numer- 
ically negative, of course, Venus sometimes being as 
bright as magnitude — 4.3, and the sun — ^26.7. 

Knowing thus the relation of sun, moon, and 
stars, and the number of the stars of different mag- 
nitudes, it is possible to estimate the total light from 
the stars. This interesting relation comes out this 
way: that the stars we cannot see with the naked 
eye give a greater total of light than those we can 
because of their vastly greater numbers. And if 
we calculate the total light of all the brighter stars 
down to magnitude nine and one-half, we find it 
equal to l/80th of the light of the average full moon. 

Many stars show marked differences in color, and 
strictly speaking the stars are now classified by 
their colors. The atmosphere affects star colors 
very considerably, low altitudes, or greater thick- 
ness of air, absorbing the bluish rays more strongly 
and making the stars appear redder than they really 
are. Aldebaran, Betelgeuse and Antares are well- 
known red stars, Capella and Alpha Ceti yellowish, 
Vega and Sirius blue, and Procyon and Polaris 
white. Among the telescopic stars are many of a 
deep blood-red tint, variable stars being numerous 
among them. Double stars, too, are often comple- 
mentary in color. There is evidence indicating 



THE UNIVERSE OF STARS 297 

change of color of a very few stars in long periods 
of time ; Sirius, for example, two thousand years ago 
was a red star, now it is blue or bluish white. But the 
meaning of color, or change of color in a star is as 
yet only incompletely ascertained. It may be con- 
nected with the radiative intensity of the star, or 
its age, or both. 

The late Professor Edward C. Pickering was 
famous for his life-long study and determination of 
the magnitudes of the stars. Standards of com- 
parison have been many, and have led to much un- 
necessary work. Pickering chose Polaris as a 
standard and devised the meridian photometer, an 
ingenious instrument of high accuracy, in which the 
light of a star is compared directly with that of the 
pole star by reflection. All the bright stars of both 
the northern and the southern skies are worked 
into a standard system of magnitudes known as 
HP, or the Harvard Photometry. 

Astronomers make use of several difl^erent kinds 
of magnitude for the stars : the apparent magnitude, 
as the eye sees it, often called the visual magnitude ; 
the photographic magnitude, as the photographic 
plate records it, and these are now determined with 
the highest accuracy; the photo visual magnitude, 
quite the same as the visual, but determined photo- 
graphically on an isochromatic plate with a yellow 
screen or filter, so that the intensity is nearly the 
same as it appears to the eye. The difference be- 
tween the star's visual or photovisual magnitude 
and its photographic magnitude is called its color- 
index, and is often used as a measure of the star's 
color. Light of the shorter wavelengths, as blue 
and violet, affects the photographic plate more 
rapidly than the reds and yellows of longer wave 



298 ASTRONOMY TO-DAY 

length by which the eye mainly sees; so that red 
stars will appear much fainter and blue stars much 
brighter on the ordinary photographic plate than 
the eye sees them. 

So great are the differences of color in the stars 
that well-known asterisms, with which the eye is 
perfectly familiar, are sometimes quite unrecogniz- 
able on the photographic plate, except by relative 
positions of the stars composing them. White stars 
affect the eye and the plate about equally, so that 
their visual or photovisual and photographic magni- 
tudes are about equal. The studies of the colors 
of the stars, the different methods of determining 
them, and the relations of color to constitution have 
been made the subject of especial investigation by 
Seares of Mount Wilson and many other astrono- 
mers. 

Centuries of the work of astronomers have been 
faithfully devoted to mapping or charting the stars 
and cataloguing them. Just as we have geographical 
maps of countries, so the heavens are parceled out 
in sections, and the stars set down in their true 
relative positions just as cities are on the map. Re- 
cent years have added photographic charts, espe- 
cially of detailed regions of the sky; but owing to 
spectral differences of the stars, their photographic 
magnitudes are often quite different from their visual 
magnitudes. From these maps and charts the 
positions of the stars can be found with much pre- 
cision; but if we want the utmost accuracy, we 
must go to the star catalogues — ^huge volumes often- 
times, with stellar positions set down therein with 
the last degree of precision. 

First there will be the star's name, and in the next 
column its magnitude, and in a third the star's 



THE UNIVERSE OF STARS 299 

right ascension. This is its angular distance east- 
ward around the celestial sphere starting from the 
vernal equinox, and it corresponds quite closely to 
the longitude of a place which we should get from a 
gazetteer, if we wished to locate it on the earth. 
Then another column of the catalogue will give the 
starts declination, north or south of the equator, 
just as the gazetteer will locate a city by its north 
or south latitude. 



CHAPTER XLV 

STAR CHARTS AND CATALOGUES 

WHO made the first star chart or catalogue? 
There is little doubt that Eudoxus (B. C. 200) 
was the first to set down the positions of all the 
brighter stars on a celestial globe, and he did this 
from observations with a gnomon and an armillary 
sphere. Later Hipparchus (B. c. 130) constructed 
the first known catalogue of stars, so that astrono- 
mers of a later day might discover what changes 
are in progress among the stars, either in their re- 
lative positions or caused by old stars disappearing 
or new stars appearing at times in the heavens. 
Hipparchus was an accurate observer, and he dis- 
covered an apparent and perpetual shifting of the 
vernal equinox westward, by which the right as- 
censions of the stars are all the time increasing. 
He determined the amount of it pretty accurately, 
too. His catalogue contained 1,080 stars, and is 
printed in the "Almagest" of Ptolemy. 

Centuries elapsed before a second star catalogue 
was made, by Ulugh-Beg, an Arabian astronomer, 
A. D. 1420, who was a son of Tamerlane, the Tartar 
monarch of Samarcand, where the observations for 
the catalogue were made. The stars were mainly 
those of Ptolemy, and much the same stars were re- 
observed by Tycho Brahe (A. D. 1580) with his 
greatly improved instruments, thus forming the 

300 



STAR CHARTS AND CATALOGUES 301 

third and last star catalogue of importance before 
the invention of the telescope. 

From the end of the seventeenth century onward, 
the application of the telescope to all the types of in- 
struments for making observations of star places 
has increased the accuracy manyfold. The entire 
heavens has been covered by Argelander in the 
northern hemisphere, and Gould in the southern — 
over 700,000 stars in all. Many government observ- 
atories are still at work cataloguing the stars. The 
Carnegie Institution of Washington maintains a 
department of astrometry under Boss of Albany, 
which has already issued a preliminary catalogue 
of more than 6,000 stars, and has a great general 
catalogue in progress, together with investigations 
of stellar motions and parallaxes. This catalogue 
of star positions will include proper motions of 
stars to the seventh magnitude. 

In 1887 on proposal of the late Sir David Gill, 
an international congress of astronomers met at 
Paris and arranged for the construction of a photo- 
graphic chart of the entire heavens, allotting the 
work to eighteen observatories, equipped with photo- 
graphic telescopes essentially alike. The total num- 
ber of plates exceeds 25,000. Stars of the fourteenth 
magnitude are recorded, but only those including 
the eleventh magnitude will be catalogued, perhaps 
2,000,000 in all. The expense of this comprehensive 
map of the stars has already exceeded $2,000,000, 
and the work is now nearly complete. Turner of 
Oxford has conducted many special investigations 
that have greatly enhanced the progress of this in- 
ternational enterprise. 

Other great photographic star charts have been 
carried through by the Harvard Observatory, with 



802 ASTRONOMY TO-DAY 

the annex at Arequipa, Peru, employing the Bruce 
photographic telescope, a doublet with 24-inch 
lenses; also Kapteyn of Groningen has catalogued 
about 300,000 stars on plates taken at Cape Town. 
Charting and cataloguing the stars, both visually 
and photographically, is a work that will never be 
entirely finished. Improvements in processes will 
be such that it can be better done in the future than 
it is now, and the detection of changes in the fainter 
stars and investigation of their motions will neces- 
sitate repetition of the entire work from century 
to century. 

The origin of the names of individual stars is a 
question of much interest. The constellation figures 
form the basis of the method, and the earliest names 
were given according to location in the especial 
figure ; as for instance, Cor Scorpii, the heart of the 
Scorpion, later known as Antares or Alpha Scorpii. 
The Arabians adopted many star names from the 
Greeks, and gave about a hundred special names 
to other stars. Some of these are in common use 
to-day, by navigators, observers of meteors and of 
variable stars. Sirius, Vega, Arcturus, and a few 
other first magnitude stars, are instances. 

But this method is quite insufficient for the 
fainter stars whose numbers increase so rapidly. 
Bayer, a contemporary of Galileo, originated our 
present system, which also employs the names of the 
constellations, the Latin genitive in each case, pre- 
fixed by the small letters of the Greek alphabet, 
from alpha to omega, in order of decreasing bright- 
ness; and followed by the Roman letters when the 
Greek alphabet is exhausted. 

If there were still stars left in a constellation un- 
iiamed, numbers were used, first by Flamsteed, 



STAR CHARTS AND CATALOGUES 303 

Astronomer Royal; and numbers in the order of 
right ascension in various catalogues are used to 
designate hundreds of other stars. The vast bulk 
of the stars are, however, nameless; but about one 
million are identifiable by their positions (right 
ascension and declination) on the celestial sphere. 



CHAPTER XL VI 

THE SUN'S MOTION TOWARD LYRA 

IF Hipparchus or Galileo should return to earth 
to-night and look at the stars and constellations 
as we see them, there would be no change whatever 
discernible in either the brightness of the stars or in 
their relative positions. So the name fixed stars 
would appear to have been well chosen. Halley in the 
seventeenth century was the first to detect that slow 
relative change of position of a few stars which is 
known as proper motion, and all the modern cata- 
logues give the proper motions in both right as- 
cension and declination. These are simply the 
small annual changes in position athwart the line 
of vision ; and, as a whole, the proper motions of the 
brighter stars exceed the corresponding motions of 
the fainter ones because they are nearer to us. The 
average proper motion of the brightest stars is 
0".25, and of stars of the sixth magnitude only one- 
sixth as great. 

A few extreme cases of proper motion have been 
detected, one as large as 9", of .an orange yellow 
star of the eighth magnitude in the southern con- 
stellation Pictor, and Barnard has recently dis- 
covered a star with a proper motion exceeding 10" ; 
several determinations of its parallax give 0".52, 
corresponding to a distance of 6.27 light years. 
Nevertheless, two centuries would elapse before 
these stars would be displaced as much as the 

304 



SUN'S MOTION TOWARD LYRA 305 

breadth of the moon among their neighbors in the 
sky. The proper motions of stars are along per- 
fectly straight lines, so far as yet observed. Ulti- 
mately we may find a few moving in curved paths 
or orbits, but this is hardly likely. 

As for a central sun hypothesis, that pointing out 
Alcyone in particular, there is no reliable evidence 
whatever. Analysis of the proper motions of stars 
in considerable numbers, first by Sir William Her- 
schel, showed that they were moving radially from 
the constellation Hercules, and in great numbers 
also toward the opposite side of the stellar sphere. 
Later investigation places this point, called the 
sun's goal, or apex of the sun's way, over in the 
adjacent constellation Lyra; and the opposite point, 
or the sun's quit, is about halfway between Sirius 
and Canopus. By means of the radial velocities of 
stars in these antipodal regions of the sky, it is 
found that the sun's motion toward Lyra, carrying 
all his planetary family along with him, is taking 
place at the rate of about 12 miles in every second. 

While the right ascensions of the solar apex as 
given by the different investigations have been 
pretty uniform, the declination of this 'point has 
shown a rather wide variation not yet explained. 
For example, there is a difference of nearly ten 
degrees between the declination (+34°. 3) of the 
apex as determined by Boss from the proper mo- 
tions of more than 6,000 stars, and the declination 
(+25°. 3) found by Campbell from the radial velo- 
cities of nearly 1,200 stars. Several investigations 
tend to show that the fainter the stars are, the 
greater is the declination of the solar apex. More 
remarkable is the evidence that this declination 
varies with the spectral type of the stars, the later 



306 ASTRONOMY TO-DAY 

types, especially G and K, giving much more north- 
erly values. On the whole the great amount of re- 
search that has been devoted to the solar motion 
relative to the system of the stars for the past 
hundred years may be said to indicate a point in 
right ascension 18^ (270°) and declination 34° N. 
as the direction toward which the sun is moving. 
This is not very far from the bright star Alpha 
Lyrae, and the antipodal point from which the sun 
is traveling is quite near to Beta Columbse. 

So swift is this motion (nearly twenty kilometers 
per second) that it has provided a base line of excep- 
tional length, and very great service in determining 
the average distance of stars in groups or classes. 
After thousands of years the sun's own motion com- 
bined with the proper motions of the stars will dis- 
place many stars appreciably from their familiar 
places. The constellations as we know them will 
suffer slight distortions, particularly Orion, Cassio- 
peia and Ursa Major. Identity or otherwise of 
spectra often indicates what stars are associated 
together in groups, and their community of motion 
is known as star drift. Recent investigation of vast 
numbers of stars by both these methods have led to 
the epochal discovery of star streaming, which in- 
dicates that the stars of our system are drifting by, 
or rather through, each other, in two stately and 
interpenetrating streams. The grand primary 
cause underlying this motion is as yet only surmised. 



\ CHAPTER XLVII 

STARS AND THEIR SPECTRAL TYPE 

WHEN in 1872 Dr. Henry Draper placed a very 
small wet plate in the camera of his spectro- 
scope and, by careful following, on account of the 
necessarily long exposure, secured the first photo- 
graphic spectrum of a star ever taken, he could 
hardly have anticipated the wealth of the new field 
of research which he was opening. His wife, Anna 
Palmer Draper, was his enthusiastic assistant in 
both laboratory and observatory, and on his death in 
1882, she began to devote her resources very con- 
siderably to the amplification of stellar spectrum 
photography. At first with the cooperation of 
Professor Young of Princeton, and later through 
extension of the facilities of Harvard College 
Observatory, whose director, the late Professor 
Edward C. Pickering, devoted his energies in very 
large part to this matter, all the preliminaries of 
the great enterprise were worked out, and a compre- 
hensive program was embarked upon, which cul- 
minated in the "Henry Draper Memorial," a cata- 
logue and classification of the spectra of all the stars 
brighter than the ninth magnitude, in both the 
northern and southern hemispheres. 

One very remarkable result from the investiga- 
tion of large numbers of stars according to their 
type is the close correlation between a star's lumi- 
nosity and its spectral type. But even more remark- 

307 



308 ASTRONOMY TO-DAY 

able is the connection between spectral type and 
speed of motion. As early as 1892 Monck of Dublin, 
later Kapteyn, and still later Dyson, directed atten- 
tion to the fact that stars of the Secchi type II had 
on the average larger proper motions than those of 
type I. In 1903 Frost and Adams brought out the 
exceptional character of the Orion stars, the radial 
velocities of twenty of which averaged only seven 
kilometers per second. 

Soon after, with the introduction of the two- 
stream hypothesis, a wider generalization was 
reached by Campbell and Kapteyn, whose radial ve- 
locities showed that the average linear velocity in- 
creases continually through the entire series B, A, 
F, G, K, M, from the earliest types of evolution to 
the latest. The younger stars of early type have 
velocities of perhaps five or six kilometers per sec- 
ond, while the older stars of later type have velocities 
nearly fourfold greater. 

The great question that occurs at once is : How do 
the individual stars get their motions? The farther 
back we go in a star's life history, the smaller we 
find its velocity to be. When a star reaches the 
Orion stage of development, its velocity is only one- 
third of what it may be expected to have finally. 
Apparently, then, the stars at birth have no motion, 
but gradually acquire it in passing through their 
several types or stages of development. 

More striking still is the motion of the planetary 
nebulae, in excess of 25 kilometers per second, while 
type A stars move 11 kilometers, type G 15 kilome- 
ters, and type M 17 kilometers per second. Can the 
law connecting speed of motion and spectral type 
be so general that the planetary nebula is to be re- 
garded as the final evolutionary stage? Stars have 



STARS AND THEIR SPECTRAL TYPE 309 

been seen to become nebulse, and one astronomer at 
least is strongly of the opinion that a single such 
instance ought to outweigh all speculation to the 
contrary, as that stars originate from nebulse. 

In his discussion of stellar proper motions, Boss 
has reached a striking confirmation of the relation 
of speed to type, finding for the cross linear motion 
of the different types a series of velocities closely 
paralleling those of Kapteyn and Campbell. 

Concerning the marked relation of the luminosi- 
ties of the stars to their spectral types, there is a 
pronounced tendency toward equality of brightness 
among stars of a given type; also the brightness 
diminishes very markedly with advance in the stage 
of evolution. There has been much discussion as to 
the order of evolution as related to the type of 
spectrum^ and Russell of Princeton has put forward 
the hypothesis of giant stars and dwarf stars, each 
spectral type having these two divisions, though not 
closely related. One class embraces intensely lumi- 
nous stars, the other stars only feebly luminous. 
When a star is in process of contraction from a 
diffused gaseous mass, its temperature rises, accord- 
ing to Lane's law, until that density is reached where 
the loss of heat by radiation exceeds the rise in tem- 
perature due to conversion of gravitational energy 
into heat. Then the star begins to cool again. So 
that if the spectrum of a star depends mainly on the 
effective temperature of the body, clearly the classi- 
fication of the Draper catalogue would group stars 
together which are nearly alike in temperature, 
taking no note as to whether their present temper- 
ature is rising or falling. 

Another classification of stars by Lockyer divides 
them according to ascending and descending tern- 



310 ASTRONOMY TO-DAY 

peratures. Russell's theory would assign the suc- 
cession of evolutionary types in the order, Mi, Ki, 
Gi, Fi, Ai, B, A2, F2, G2, Kz, M2, the subscript 1 re- 
ferring to the "giants/* and 2 to the dwarf stars. 
In large part the weight of evidence would appear 
to favor the order of the Harvard classification, in- 
dependently confirmed as it is by studies of stellar 
velocities. Galactic distribution, and periods of bi- 
nary stars both spectroscopic and visual, where 
Campbell and Aiken find a marked increase in 
length of period with advance in spectral type. At 
the same time, a vast amount of evidence is accumu- 
lating in support of Russell's theory. Investigations 
in progress will doubtless reveal the ground on 
which both may be harmonized. 

The publication of the new Henry Draper Cata- 
logue of Stellar Spectra is in progress, a work of 
vast magnitude. The great catalogue of thirty 
years ago embraced the spectra of more than ten 
thousand stars, and was a huge work for that day; 
but the new catalogue utterly dwarfs it, with a 
classification much more detailed than in the 
earlier work, and with the number of stars increased 
more than twentyfold. This work, projected by the 
late director of the Harvard Observatory, has been 
brought to a conclusion by the energy and enthu- 
siasm of Miss Annie J. Cannon through six years of 
close application, aided by many assistants. The 
catalogue ranges over the stars of both hemispheres, 
and is a monument to masterly organization and 
completed execution which will be of the highest 
importance and usefulness in all future researches 
on the bodies of the stellar universe. 



CHAPTER XLVIII 

STAR DISTANCES 

SO vast are the distances of the stars that all at- 
tempts of the early astronomers to ascertain 
them necessarily proved futile. This led many as- 
tronomers after Copernicus to reject his doctrine 
of the earth's motion round the sun, so that they 
clung rather to the Ptolemaic view that the earth 
was without motion and was the center about which 
all the celestial motions took place. The geometry 
of stellar distances was perfectly understood, and 
many were the attempts made to find the parallaxes 
and distances of the stars ; but the art of instrument 
making had not yet advanced to a stage where 
astronomers had the mechanisms that were abso- 
lutely necessary to measure very small angles. 

About 1835, Bessel undertook the work of deter- 
mining stellar parallax in earnest. His instrument 
was the heliometer, originally designed for measur- 
ing the sun's diameter ; but as modified for parallax 
work it is the most accurate of all angle-measuring 
instruments that the astronomers employ. The star 
that he selected was 61 Cygni, not a bright star, 
of the sixth magnitude only, but its large proper 
motion suggested that it might be one of those 
nearest to us. He measured with the heliometer, 
at opposite seasons of the year, the distance of 61 
Cygni from another and very small star in the 
same field of view, and thus determined the relative 

311 



312 ASTRONOMY TO-DAY 

parallax of the two stars. The assumption was 
made that the very faint star was very much more 
distant than the bright one, and this assumption 
will usually turn out to be sound. Bessel got 0".35 
for his parallax of 61 Cygni, and Struve by apply- 
ing the same method to Alpha Lyrse, about the 
same time, got 0".25 for the parallax of that star. 

These classic researches of Bessel and Struve 
are the most important in the history of star dis- 
tances, because they were the first to prove that 
stellar parallax, although minute, could neverthe- 
less be actually measured. About the same time 
success was achieved in another quarter, and Hen- 
derson, the British astronomer at the Cape of Good 
Hope, found a parallax of nearly a whole second 
for the bright star Alpha Centauri. 

Although the parallaxes of many hundreds of 
stars have been measured since, and the parallaxes 
of other thousands of stars estimated, the measured 
parallax of Alpha Centauri, as later investigated by 
Elkin and Sir David Gill, and found to be 0".75, 
is the largest known parallax, and therefore Alpha 
Centauri is our nearest neighbor among the stars, 
so far as we yet know. This star is a binary system 
and the light of the two components together is 
about the same as that of Capella (Alpha Aurigse). 
But it is never visible from this part of the world, 
being in 60 degrees of south declination : one might 
just glimpse it near the southern horizon from Key 
West. 

How the distances of the stars are found is not 
difficult to explain, although the method of doing it 
involves a good deal of complication, interesting to 
the practical astronomer only. Recall the method of 
getting the moon's distance from the earth: it was 



STAR DISTANCES 313 

done by measuring her displacement among the stars 
as seen from two widely separated observatories, 
as near the ends of a diameter of the earth as con- 
venient. This is the base line, and the angle 
which a radius of the earth as seen from the 
center of the moon fills, or subtends, is the moon's 
parallax. 

So near is the moon that this angle is almost 
an entire degree, and therefore not at all difficult 
to measure. But if we go to the distance of even 
Alpha Centauri, the nearest of the stars, our earth 
shrinks to invisibility; so that we must seek a 
longer base line. Fortunately there is one, but 
although its length is 25,000 times the earth's diam- 
eter, it is only just long enough to make the star 
distances measurable. We found that the sun's dis- 
tance from the earth was 93 million miles; the 
diameter of the earth's orbit is therefore double 
that amount. Now conceive the diameter of the 
earth replaced by the diameter of the earth's 
orbit: by our motion round the sun we are trans- 
ported from one extremity of this diameter to the 
opposite one in six month's time; so we may mea- 
sure the displacement of a star from these two ex- 
tremities, and half this displacement will be the 
star's parallax, often called the annual parallax 
because a year is consumed in traversing its period. 
And it is this very minute angle which Bessel and 
Struve were the first to measure with certainty, 
and which Henderson found to be in the case of 
Alpha Centauri the largest yet known. 

Evidently the earth by its motion round the sun 
makes every star describe a little parallactic ellipse ; 
the nearer the star is the larger this ellipse will 
be, and the farther the star the smaller ; if the star 



314 ASTRONOMY TO-DAY 

were at an infinite- distance, its ellipse would be- 
come a point, that is, if we imagine ourselves 
occupying the position of the star, even the vast 
orbit of the earth, 186 million miles across, would 
shrink to invisibility or become a mathematical 
point. 

Measurement of stellar parallax is one of many 
problems of exceeding difficulty that confront the 
practical astronomer. But the actual research now- 
adays is greatly simplified by photography, which 
enables the astronomer to select times when the air 
is not only clear, but very steady for making the 
exposures. Development and measurement of the 
plates can then be done at any time. Pritchard of 
Oxford, England, was among the earliest to appre- 
ciate the advantages of photography in parallax 
work, and Schlesinger, Mitchell, Miller, Slocum 
and Van Maanen, with many others in this country, 
have zealously prosecuted it. 

How shall we intelligently express the vast dis- 
tances at which the stars are removed from us? 
Of course we can use miles, and pile up the millions 
upon millions by adding on ciphers, but that fails to 
give much notion of the star's distance. Let us try 
with Alpha Centauri: its parallax of 0".75 means 
that it is 275,000 times farther from the sun than 
the earth is. Multiplying this out, we get 25 tril- 
lion miles, that is, 25 millions of million miles 
— an inconceivable number, and an unthinkable 
distance. 

Suppose the entire solar system to shrink so that 
the orbit of Neptune, sixty times 93 million miles 
in diameter, would be a circle the size of the dot 
over this letter i. On the same scale the sun itself, 
although nearly a million miles in diameter, could 



STAR DISTANCES 315 

not be seen with the most powerful microscope in 
existence; and on the same scale also we should 
have to have a circle ten feet in diameter, if the 
solar system were imagined at its center and Alpha 
Centauri in its circumference. 

So astronomers do not often use the mile as a 
yardstick of stellar distance, any more than we 
state the distance from London to San Francisco 
in feet or inches. By convention of astronomers, 
the average distance between the centers of sun 
and earth, or 93 million miles, is the accepted 
unit of measure in the solar system. So the adopted 
unit of stellar distance is the distance traveled by 
a wave of light in a year's time: and this unit is 
technically called the light-year. This unit of dis- 
tance, or stellar yardstick, as we may call it, is 
nearly 6 millions of million miles in length. Alpha 
Centauri, then, is four and one-third light-years 
distant, and 61 Cygni seven and one-fifth light- 
years away. 

For convenience in their calculations most as- 
tronomers now use a longer unit called the parsec, 
first suggested by Turner. Its length is equal to the 
distance of a star whose parallax is one second of 
arc; that is, one parsec is equal to about three 
and a quarter light-years. Or the light-year is 
equal to 0.31 parsec. Also the parsec is equal to 
206,000 astronomical units, or about 19 millions of 
million miles. 

We have, then four distinct methods of stating 
the distance of a star: Sirius, for example, has a 
parallax of 0".38 or its distance is two and two- 
thirds parsecs, or eight and a half light-years, or 
50 millions of milHon miles. It is the angle of 
parallax which is always found first by actual meas- 



316 ASTRONOMY TO-DAY 

urement and from this the three other estimates 
of distance are calculated. 

So difficult and delicate is the determination of 
a stellar distance that only a few hundred paral- 
laxes have been ascertained in the past century. 
The distance of the same star has been many times 
measured by different astronomers, with much 
seeming duplication of effort. Comprehensive cam- 
paigns for determining star parallaxes in large 
numbers have been undertaken in a few instances, 
particularly at the suggestion of Kapteyn, the em- 
inent astronomer of Groningen, Holland. His cata- 
logue of star parallaxes is the most complete and 
accurate yet published, and is the standard in all 
statistical investigations of the stars. 

That we find relatively large parallaxes for 
some of the fainter stars, and almost no measur- 
able parallax for some of the very bright stars is 
one of the riddles of the stellar universe. We may 
instance Arcturus, in the northern hemisphere and 
Canopus in the southern; the latter almost as 
bright as Sirius. Dr. Elkin and the late Sir David 
Gill determined exhaustively the parallax of Cano- 
pus, and found it very minute, only 0".03, making 
its distance in excess of a hundred light-years. The 
stupendous brilliancy of this star is apparent if we 
remember that the intensity of its light must vary 
inversely as the square of the distance; so that if 
Canopus were to be brought as near us as even 
61 Cygni is, it would be a hundredfold brighter 
than Sirius, the brightest of all the stars of the 
firmament. 

In researches upon the distribution of the more 
distant stars, the method of measuring parallaxes 
of individual stars fails completely, and the secular 



STAR DISTANCES 317 

parallax, or parallactic motion of the stars is em- 
ployed instead. By parallactic motion is meant the 
apparent displacement in consequence of the solar 
motion which is now known with great accuracy, 
and amounts to 19.5 kilometers per second. Even 
in a single year, then, the sun's motion is twice the 
diameter of the earth's orbit, so that in a hundred 
or more years, a much longer base line is available 
than in the usual type of observations for stellar 
parallax. If we ascertain the parallactic motion of 
a group of stars, then we can find their average 
distance. It is found, for example, that the mean 
parallax of stars of the sixth magnitude is 0".014. 
Also the mean distances of stars throv^ni into 
classes according to their spectral type have been 
investigated by Boss, Kapteyn, Campbell and 
others. The complete intermingling of the two 
great star streams has been proved, too, by using 
the magnitude of the proper motions to measure 
the average distances of both streams. These come 
out essentially the same, so that the streaming can- 
not be due to mere chance relation in the line of 
sight. 

Most unexpected and highly important is the dis- 
covery that the peculiar behavior of certain lines 
in the spectrum leads to a fixed relation between a 
star's spectrum and its absolute magnitude, which 
provides a new and very effective method of ascer- 
taining stellar distances. By absolute magnitudes 
are meant the magnitudes the stars would appear to 
have if they were all at the same standard distance 
from the earth. 

Very satisfactory estimates of the distance of 
exceedingly remote objects have been made within 
recent years by this indirect method, which is espe- 



318 ASTRONOMY TO-DAY 

cially applicable to spiral nebulse and globular clus- 
ters. The absolute magnitude of a star is inferred 
from the relative intensities of certain lines in its 
spectrum, so that the observed apparent magnitude 
at once enables us to calculate the distance of the 
star. Adams and Joy have recently determined 
the luminosities and parallaxes of 500 stars by this 
spectroscopic method. Of these stars 360 have had 
their parallaxes previously measured; and the 
average difference between the spectroscopic and 
the trigonometric values of the parallax is only 
the very small angle 0".0037, a highly satisfactory 
verification. 

An indirect method, but a very simple one, and 
of the greatest value because it provides the key 
to stellar distances with the least possible calcula- 
tion, and we can ascertain also the distances of 
whole classes of stars too remote to be ascertained 
in any other way at present known. 

The problem of spectroscopic determinations of 
luminosity and parallax has been investigated at 
Mount Wilson with great thoroughness from all 
sides, the separate investigations checking each 
other. A definitive scale for the spectroscopic de- 
termination of absolute magnitudes has now been 
established, and the parallaxes and absolute magni- 
tudes have already been derived for about 1,800 
stars. 



CHAPTER XLIX 

THE NEAREST STARS 

OF especial interest are the few stars that we 
know are the nearest to us, and the following 
table includes all those whose parallax is 0".20 or 
greater. There are nineteen in all and nearly half 
of them are binary systems. The radial motions 
given are relative to the sun. The transverse veloc- 
ities are formed by using the measured parallaxes 
to transform proper motions into linear measures. 
They are given by Eddington in his "Stellar Move- 
ments" : 



star's Name 


t 


111 




ill 


Radial 

Velocity 

Km. per sec. 


V 


1?^ 


1 


Groombridge 34 . . 

Eta Cassiop 

Tau Cetl 

Epsilon Brid 

CZ 5h 243 

Sirius 


8.2 
3.6 
3.6 
3.3 
8.3 
-1.6 
0.5 
7.6 
8.9 
9.2 

0.3 

9.3 

8.8 
4.8 
0.9 
5.6 
4.7 
9.2 
7.4 


0.28 
0.20 
0.33 
0.31 
0.32 
0.38 
0.32 
0.40 
0.20 
0.20 

0.76 

0.27 
0.29 
0.20 
0.24 
0.31 
0.28 
0.26 
0.29 


2.85 
1.25 
1.93 
1.00 
8.70 
1.32 
1.25 
4.77 
4.46 
3.03 

3.66 

1.31 
2.28 
1.84 
0.65 
5.25 
4.67 
0.92 
7.02 


48 
30 
28 
15 

129 
16 
19 
57 

106 
72 

23 

23 
37 
43 
13 
80 
79 
17 
115 


+10 

-16 

+16 

+242 

—7 

-3 

-22 

+25 
-33 
-39 
-62 

+12 


Ma 

P8 

K 

K 

G-K 

A 

P5 

Ma 

Ma 

G.K5 

F 

K 

K 

A5 

K5 

K5 

Ma 


0.010 

1.4 

0.50 

0.79 

0.007 

48.0 
9.7 
0.009 
0.011 
0.008 

52.0 

(0.6 
0.004 
0.006 
0.5 

12.3 
0.10 
0.25 
0.005 
0.019 




Procyon 

Lai. 21185 

Lai. 21258 

OA (N) 11677.. 

Alpha Centauri. 

OA (N) 17415. . 
Pos. Med. 2164. . 

Sigma Draco 

Alpha Aquilas . . . 

61 Cygni 

Epsilon Indi 

Kriiger 60 
Lacaille 9352 





319 



320 ASTRONOMY TO-DAY 

These stars are distant less than five parsecs 
(about 16 light-years) from the sun, so they make 
up the closest fringe of the stellar universe im- 
mediately surrounding our system. The large num- 
ber of binary systems is quite remarkable. Why 
some stars are single and others double is not yet 
known. By the spectroscopic method the propor- 
tion is not so large; Campbell finding that about 
one quarter of 1,600 stars examined are spectro- 
scopic binaries, and Frost two-fifths to a half. The 
exceptional number of large velocities is very re- 
markable; the average transverse motion of the 
nineteen stars is fifty kilometers per second, where- 
as thirty is about what would have been expected. 

As to star streams to which these nearest stars 
belong, eleven are in Stream I and eight in Stream 
II, in close accord with the ratio 3 :2 given by the 
'6,000 stars of Boss's catalogue. "We are not able," 
says Eddington, "to detect any significant difference 
between the luminosities, spectra, or speeds of the 
stars constituting the two streams. The thorough 
interpenetration of the two star streams is well 
illustrated, since we find even in this small volume 
of space that members of both streams are mingled 
together in just about the average proportion." 




The Ring Nebula in Lyra. This is the best example of the annular and 

elliptic nebulae, which are not very abundant. {Photo, Mt. Wilson Sola7- 

Observatory.) 



CHAPTER L 

ACTUAL DIMENSIONS OF THE STARS 

WE have seen that the distances of the stars 
from the solar system are immense beyond 
conception, and millions upon millions of them are 
probably forever beyond our power of ascertaining 
by direct measurement what their distance really 
is. After we had found the sun's distance and 
measured the angle filled by his disk, it was easy 
to calculate his actual size. This direct method, 
however, fails when we try to apply it to the stars, 
because their distances are so vast that no star's 
disk fills an angle of any appreciable size ; and even 
if we try to get a disk with the highest magnifying 
powers of a great telescope our efforts end only in 
failure. There is, indeed, no instrumentally appre- 
ciable angle to measure. 

How then shall we ascertain the actual dimen- 
sions of the vast spheres which we know the stars 
actually are, as they exist in the remotest regions 
of space? Clearly by indirect methods only, and it 
must be said that astronomers have as yet no gen- 
eral method that yields very satisfactory results for 
stellar dimensions. The actual magnitude of the 
variable system of Algol, Beta Persei, is among the 
best known of all the stars, because the spectro- 
scope measures the rate of approach and recession 
of Algol when its invisible satellite is in opposite 
parts of the orbit ; the law of gravitation gives the 

321 Sci. Vol, 2—11 



322 ASTRONOMY TO-DAY 

mass of the star and the size of its orbit, and so 
the length of the eclipse gives the actual size of 
the dark, eclipsing body. This figures out to be 
practically the same size as that of our sun, while 
Algol's own diameter is rather larger, exceeding a 
million miles. 

If we try to estimate sizes of stars by their bright- 
ness merely, we are soon astray. Differences of 
brightness are due to difference of dimensions, of 
course, or of light-giving area; but differences of 
distance also affect the brightness, inversely as the 
squares of the distances, while differences of tem- 
perature and constitution affect, in very marked 
degree, the intrinsic brilliance of the light-emitting 
surface of the star. There are big stars and little 
stars, stars relatively near to us and stars exceed- 
ingly remote, and stars highly incandescent as well 
as others feebly glowing. 

We have already shown how the angular diam- 
eters subtended by many of the stars have been 
estimated, through the relation of surface bright- 
ness and spectral type. Antares and Betelgeuse 
appear to be the most inviting for investigation, be- 
cause their estimated angular diameters are about 
one-twentieth of a second of arc. This is the 
way in which their direct measurement is being 
attempted. 

As early as 1890, Michelson of Chicago suggested 
the application of interference methods to the ac- 
curate measurement of very small angles, such as 
the diameters of the minor planets, and the satel- 
lites of Jupiter and Saturn, as well as the arc dis- 
tance between the components of double stars. Two 
portions of the object glass are used, as far apart 
as possible on the same diameter, and the inter- 



ACTUAL DIMENSIONS OF STARS 323 

ference fringes produced at the focus of the objec- 
tive are then the subject of observation. These 
fringes form a series of equidistant interference 
bands, and are most distinct when the light comes 
from a source subtending an infinitesimal angle. 
If the object presents an appreciable angle, the vis- 
ibility is less and may even become zero. 

Michelson tested this method on the satellites of 
Jupiter at the Lick Observatory in 1891, and 
showed its accuracy and practicability. Neverthe- 
less, the method has not been taken up by astron- 
omers, until very recently at the Mount Wilson 
Observatory, where Anderson has applied it to the 
measurement of close double stars. It is found that, 
contrary to general expectation, the method gives 
excellent results, even if the '^seeing" is not the best 
— 2 on a scale of 10, for instance. 

To simplify the manipulation of the interfero- 
meter, a small plate with two apertures in it is 
placed in the converging beam of light coming from 
the telescope objective or mirror. The interference 
fringes formed in the focal plane are then viewed 
with an eyepiece of very high power, many thou- 
sand diameters. The resolving power of the inter- 
ferometer is found to be somewhat more than 
double that of a telescope of the same aperture. By 
applying the interferometer method to Capella, arc 
distances of much less than one-twentieth of a 
second of arc were measured. More recently the 
method has been applied to the great star Betelgeuse 
in Orion, whose angular diameter was found to be 
0".46, corresponding to an actual diameter of 260,- 
000,000 miles, if the star's parallax is as small as it 
appears to be. 



CHAPTER LI 

THE VARIABLE STARS 

SPECTACULAR as they are to the layman, novse, 
or temporary stars, are to the astronomers 
simply a class among many thousands of stars 
which they call variables, or variable stars. There 
are a few objects classified as irregular variables, 
one of which is very remarkable. We refer to Eta 
Argus, an erratic variable in the southern constel- 
lation Argo and surrounded by a well-known 
nebula. There is a pretty complete record of this 
star. Halley in 1677 when observing at Saint 
Helena recorded Eta Argus as of the fourth magni- 
tude. During the 18th century, it fluctuated be- 
tween the fourth magnitude and the second. Early 
in the 19th it rapidly waxed in brightness, fluctu- 
ating between the first and second magnitudes 
from 1822 to 1836. But two years later its light 
tripled, rivaling all the fixed stars except Canopus 
and Sirius. In 1843 it was even brighter for a few 
months, but since then it has declined fairly stead- 
ily, reaching a minimum at magnitude seven and a 
half in 1886, with a slight increase in brightness 
more recently. A period of half a century has been 
suggested, but it is very doubtful if Eta Argus has 
any regular period of variation. 

Another very interesting class of variables is 
known as the Omicron Ceti type. Nearly all the 
time they are very faint, but quite suddenly they 

324 



THE VARIABLE STARS 325 

brighten through several magnitudes, and then fade 
away, more or less slowly, to their normal condi- 
tion of faintness. But the extraordinary thing is 
that most of these variables go through their fluctu- 
ations in regular periods: from six months to two 
years in length. The type star, Omicron Ceti, or 
Mira, is the oldest known variable, having been 
discovered by Fabricius in 1596. Most of the time 
it is a relatively faint star of the 12th magnitude; 
but once in rather less than a year its brightness 
runs up to the fourth, third and sometimes even 
the second magnitude, where it remains for a week 
or ten days, and afterward it recedes more slowly 
to its usual faintness, the entire rise and decline 
in brightness usually requiring about 100 days. The 
spectrum of Omicron Ceti contains many very 
bright lines, and a large proportion of the variable 
stars are of this type. 

Another class of variables is designated as the 
Beta Lyrse type. Their periods are quite regular, 
but there are two or more maxima and minima of 
light in each period, as if the variation were caused 
by superposed relations in some way. Their spectra 
show a complexity of helium and hydrogen bands. 
No wholly satisfactory explanation has yet been 
offered. Probably they are double stars revolving 
in very small orbits compared with their dimen- 
sions, their plane of motion passing nearly through 
the earth. 

But the most interesting of all the variables are 
those of the Algol type, their light curves being 
just the reverse of the Omicron Ceti type; that is, 
they are at their maximum brightness most of the 
time, and then suffer a partial eclipse for a rela- 
tively brief interval. Algol goes through its varia- 



326 ASTRONOMY TO-DAY 

tions so frequently that its period is very accurately 
known ; it is 2d. 20h. 48m. 55.4s. For most of this 
period Algol is an easy second magnitude star ; then 
in about four and a half hours it loses nearly five- 
sixths of its light, receding to the fourth magni- 
tude. Here at minimum it remains for fifteen or 
twenty minutes, and then in the next three and a 
half hours it regains its full normal brilliancy of 
the second magnitude. During these fluctuations 
the star's spectrum undergoes no marked changes. 
I'he spectra of all the Algol variables are of the 
first or Sirian type. 

To explain the variation of the Algol type of 
variables is easy : a dark, eclipsing body, somewhat 
smaller than the primary is supposed to be travel- 
ing round it in an orbit lying nearly edgewise to 
our line of sight. The gravitation of this dark com- 
panion displaces Algol itself alternately toward and 
from the earth, because the two bodies revolve 
round their common center of gravity. With the 
spectroscope this alternate motion of Algol, now 
advancing and now receding at the rate of 26 miles 
per second, has been demonstrated; and the period 
of this motion synchronizes exactly with the period 
of the star's variability. 

Russell and Shapley have made extended studies 
of the eclipsing binaries, and developed the for- 
mulae by which the investigations of their orbits 
are conducted. Heretofore, visual binaries and 
spectroscopic binaries afforded the only means of 
deriving data regarding double systems, but it is 
now possible to obtain from the orbits of eclipsing 
variables fully as much information relating to 
binary systems in general and their bearing on 
stellar evolution. After an orbit has been deter- 



THE VARIABLE STARS 327 

mined from the photometric data of the light curve, 
the addition of spectroscopic data often permits the 
calculation of the masses, dimensions and densities 
in terms of the sun. Shapley's original investiga- 
tion included the orbits of ninety eclipsing vari- 
ables, and with the aid of hypothetical parallaxes, 
he computed the approximate position of each 
system in space. The relation to the Milky Way is 
interesting, the condensation into the Galactic 
plane being very marked; only thirteen of the 
ninety systems being found at Galactic latitudes 
exceeding 30 degrees. 

If we can suppose the variable stars covered with 
vast areas of spots, perhaps similar to the spots on 
the sun, and then combine the variation of these 
spot areas with rotation of the star on its axis, 
there is a possibility of explanation of many of 
the observed phenomena, especially where the 
range of variation is small. But for the Omicron 
Ceti type, no better explanation offers than that 
afforded by Sir Norman Lockyer's collision theory. 
First he assumes that these stars are not con- 
densed bodies, but still in the condition of meteoric 
swarms, and the revolution of lesser swarms 
around larger aggregations, in elliptic orbits of 
greater or less eccentricity, must produce vast 
multitudes of collisions ; and these collisions, taking 
place at pretty regular periods, produce the vari- 
able maximum light by raising hosts of meteoric 
particles to a state of incandescence simultaneously. 

The catalogues of variable stars now contain 
many thousands of these objects. They are often 
designated by the letters R, S, T, and so on, fol- 
lowed by the genitive form of the name of the con- 
stellation wherein they are found. Most of the re- 



328 ASTRONOMY TO-DAY 

cently found variables have a range of less than 
one magnitude. They are so distributed as to be 
most numerous in a zone inclined about 18 degrees 
to the celestial equator, and split in two near where 
the cleft in the Galaxy is located. Nearly all the 
temporary stars are in this duplex region. Bailey 
of Harvard a quarter century ago began the inves- 
tigation of variables in close star clusters, where 
they are very abundant, with marked changes of 
magnitude within only a few hours. 

Many amateur astronomers afford very great 
assistance to the professional investigator of vari- 
able stars by their cooperation in observing these 
interesting bodies, in particular the American As- 
sociation of Observers of Variable Stars, organized 
and directed by William Tyler Olcott. 

For a high degree of accuracy in determining 
stellar magnitudes the photo-electric cell is unsur- 
passed. Stebbins of Urbana has been very success- 
ful in its application and he discovered the second- 
ary minimum of Algol with the selenium cell. His 
most recent work was done with a potassium cell 
with walls of fused quartz, perfected after many 
trial attempts. The stars he has recently investi- 
gated are Lambda Tauri, and Pi Five Orionis. Com- 
bining results with those reached by the spectro- 
scope, the masses of the two component stars of 
the former are 2.5 and 1.0 that of the sun, and 
the radii are 4.8 and 3.6 times the sun's. 

Russell of Princeton thinks it probable that sim- 
ilar causes are at work in all these variables. In the 
case of the typical Novse there is evidence that 
when the outburst takes place a shell of incandes- 
cent gas is actually ejected by the star at a very 
high velocity. What may be the forces that cause 



THE VARIABLE STARS 329 

such an explosion can only be guessed. Repeated 
outbursts have not, in the case of T Pyxidis, de- 
stroyed the star, because it has gone through this 
process three times in the past thirty years. Rus- 
sell inclines to regard it as a standard process 
occurring somewhere in the stellar universe prob- 
ably as often as once a year. 

Novse, then, cannot be due to collisions between 
two stars, for even if we suppose the stars to be 
a thousand millions in number, no two should col- 
lide except at average intervals of many million 
years. The idea is gaining ground that the stars 
are vast storehouses of energy which they are 
gradually transforming into heat and radiating 
into space. "Under ordinary circumstances, it is 
probable that the rate of generation of heat is 
automatically regulated to balance the loss by radi- 
ation. But it is quite conceivable that some sudden 
disturbance in the substance of the star, near the 
surface, might cause an abrupt liberation of a 
great amount of energy, sufficient to heat the sur- 
face excessively, and drive the hot material off 
into infinite space, in much the form of a shell of 
gas, as seems to have been observed in the case of 

Nova Aquilse With the rapid advance of our 

knowledge of the properties of the stars on one 
hand, and of the very nuclei of atoms on the other, 
we may, perhaps before many years have passed, 
find ourselves nearer a solution of the problem." 

The Cepheid variables increase very rapidly in 
brightness from their least light to their maximum, 
and then fade out much more slowly, with certain 
irregularities or roughnesses of their light-curves 
when declining. Their spectral lines also shift in 
period with their variations of light. In the case 



S3Q ASTRONOMY TO-DAY 

of these variables, whose regular fluctuation of 
light cannot be due to eclipse, and is as a rule 
embraced within a few days, there is a fluctuation 
in color also between maximum and minimum, as 
if there were a periodic change in the star's physi- 
cal condition. Eddington and Shapley advocate the 
theory of a mechanical pulsation of the star as most 
plausible. Knowledge of the internal conditions of 
the stars make it possible to predict the period of 
pulsation within narrow limits; and for Delta 
Cephei this theoretical period is between four and 
ten days. Its observed period is five and one4hird 
days, and corresponding agreement is found in 
all the Cepheids so far tested. 

Shapley of Mount Wilson finds that the Cepheid 
variables with periods exceeding a day in length 
all lie close to the Galactic lane. So greatly have 
the studies of these objects progressed that, as be- 
fore remarked, when we know the star's period, 
we can get its absolute magnitude, and from this 
the star's distance. On all sides of the sun, the dis- 
tances of the Cepheids range up to 4,000 parsecs. 
So they indicate the existence of a Galactic system 
far greater in extent than any previously dealt 
with. 



CHAPTER LH 

THE NOV^, OR NEW STARS 

NEW stars, or temporary stars, we have already 
mentioned in connection with variables. They 
are, next to comets, the most dramatic objects in 
the heavens. They may be variable stars which, 
in a brief period, increase enormously in bright- 
ness, and then slowly wane and disappear entirely, 
or remain of a very faint stellar magnitude. 

In the ancient historical records are found 
accounts of several such stars. For instance, in 
the Chinese annals there is an allusion to such a 
stellar outburst in the constellation of Scorpio, 
B. c. 134. This was observed also by Hipparchus 
and, no doubt, it was the immediate incentive which 
led to his construction of the first known catalogue 
of stars, so that similar happenings might be de- 
tected in the future. In November, 1572, Tycho 
Brahe observed the most famous of all new stars, 
which blazed out in the constellation Cassiopeia. 
In something over a year it had completely dis- 
appeared. 

In 1604-1005 a new star of ^ual brightness was 
seen in Ophiuchus by Kepler; it also faded out to 
invisibility in 1606. Kepler and Tycho printed 
very complete records of these remarkable objects. 
The eighteenth century passed without any new 
stars being seen or recorded. There was one of the 
fifth magnitude in 1848, and another of the seventh 

331 



332 ASTRONOMY TO-DAY 

magnitude in 1860 ; and in May, 1866, a star of the 
second magnitude suddenly made its appearance 
in Corona Borealis; and one of the third mag- 
nitude in Cygnus in November, 1876. The latter 
was fully observed by Schmidt of Athens and 
became a faint telescopic star within a few weeks. 
It is now of the fifteenth magnitude. 

In 1885 astronomers were surprised to find sud- 
denly a new star of the sixth magnitude very close 
to the brightest part of the great nebula in An- 
dromeda; it ran its course in about six months, 
fading with many fluctuations in brightness, and 
no star is now visible in its position even with 
the telescope. Stars of this class are known to 
astronomers as Novae, usually with the genitive of 
the constellation name, as Nova Andromedse. 

In 1891-1892 Nova Aurigse made its spectacular 
appearance and yielded a distinctly double and 
complex spectrum for more than a month. Many 
pairs of lines indicated a community of origin as 
to substance, and accurate measurement showed 
a large displacement with a relative velocity of 
more than 500 miles per second. For each bright 
hydrogen line displaced toward the red there was 
a dark companion line or band about equally dis- 
placed toward the volet : much as if the weird light 
of Nova Aurigae originated in a solid globe moving 
swiftly away from us and plunging into an irregu- 
lar nebulous mass as swiftly approaching us. 
Parallax observations of Nova Aurigse made it im- 
mensely remote, perhaps within the Galaxy, and it 
still exists as a faint nebulous star. 

In February, 1901, in the constellation Perseus 
appeared the most brilliant nova of recent years, 
it was first discovered by Dr. Anderson, an 



THE NOV^, OR NEW STARS 333 

amateur of Glasgow, and at maximum on Febru- 
ary 23 it outshone Capella. There were many un- 
usual fluctuations in its waning brightness. Its 
spectrum closely resembled that of Nova Aurigse, 
with calcium, helium, and hydrogen lines. In 
August, 1901, an enveloping nebula was discovered, 
and a month later certain wisps of this nebulosity 
appeared to have moved bodily, at a speed seventy- 
fold greater than ever previously observed in the 
stellar universe. 

According to Sir Norman Lockyer's meteoritic 
hypothesis, a vast nebulous region was invaded, not 
by one but by many meteor swarms, under condi- 
tions such that the effects of collision varied greatly 
in intensity. The most violent of these collisions 
gave birth to Nova Persei itself, and the least 
violent occurred subsequently in other parts of the 
disturbed nebula, perhaps immeasurably removed. 
This explanation would avoid the necessity of sup- 
posing actual motion of matter through space at 
velocities heretofore unobserved and inconceivably 
high. A recent photograph of Nova Persei, by 
Ritchey, reveals a nebulous ring of regular structure 
surrounding the star. The great power of the 
60-inch has made it possible to photograph even the 
spectra of many of the novse of years ago which are 
now very faint. After the lapse of years the char- 
acteristic lines of the nebular spectrum generally 
vanish, as if the star had passed out of the nebula — 
a plunge into which is generally thought to be the 
cause of the great and sudden outburst of light. 

Many novse have recently been found in the 
spiral nebulae, especially in the great nebula of 
Andromeda. 



CHAPTER LIII 

THE DOUBLE STARS 

T7IXAMINING individual stars of the heavens more 
-" in detail, thousands of them are found to be 
double ; not the stars that appear double to the naked 
eye, as Theta Tauri, Mizar, Epsilon Lyrse, and others ; 
but pairs of stars much closer together, and requir- 
ing the power of the telescope to divide or separate 
them. Only a very few seconds apart they are or, 
in many cases, only the merest fraction of a second 
of arc. Some of them, called binaries, are found to 
be revolving around a common center, sometimes in 
only a few years, sometimes in stately periods of 
hundreds of years. Many such binary systems are 
now known, and the number is constantly in- 
creasing. Castor is one. Gamma Virginis another, 
Sirius also is one of these binaries, and a most 
interesting one, having a period of revolution of 
about 52 years. 

Aitken, of the Lick Observatory, in his work on 
binary stars, directs special attention to the cor- 
relation between the elements of known binary 
orbits and the star's spectral type, and presents 
a statistical study of the distribution of 54,000 
visual double stars, of which the spectra of 3919 
are known. That the masses of binary systems 
average about twice that of the sun's mass has long 
been known, and this fact can be employed with 
confidence in estimates of the probable parallax of 

334 



THE DOUBLE STARS 335 

these systems. Aitken applies the test to fourteen 
visual systems for which the necessary data are 
available, and deduces for them a mean mass of 
1.76 times that of the sun. For the spectroscopic 
binaries the masses are much greater. 

Triple, quadruple and multiple stars are less 
frequent; but many exceedingly interesting objects 
of this class exist. Epsilon Lyrse is one, a double- 
double, or four stars as seen v^^ith slender telescopic 
power, and six or seven stars with larger instru- 
ments. Sigma Orionis and 12 Lyncis, also Theta 
Cancri and Mu Bootis are good examples of triple 
stars. 



CHAPTER LIV 

THE STAR CLUSTERS 

FROM multiple stars the transition is natural to 
star clusters although the gap between these types 
of stellar objects is very broad. The familiar group 
of the winter sky known as the Pleiades is a loose 
cluster, showing relatively very few stars even in 
telescopes or on photographic plates. The "Bee- 
hive/' or cluster known as Praesepe in Cancer, and 
a double group in the sword-handle of Perseus, both 
just visible to the naked eye, are excellent examples 
of star clusters of the average type. When the moon 
is absent, they are easily recognized without a tele- 
scope as little patches of nebulous light; but every 
increase of optical power ^adds to their magnificence. 
Then we come in regular succession to the truly 
marvelous globular clusters, that for instance in 
Hercules. Messier 13, a recent photograph of 
which, taken by Ritchey with the 60-inch reflector 
on Mount Wilson, reveals an aggregation of more 
than 50,000 stars. But the finest specimens are in 
the southern hemisphere. Sir John Herschel spent 
much time investigating them nearly a century ago 
at the Cape of Good Hope. His description of the 
cluster in the constellation of Centaurus is as fol- 
lows: "The noble globular cluster Omega Centauri 
is beyond all comparison the richest and largest ob- 
ject of the kind in the heavens. The stars are liter- 
ally innumerable, and as their total light when re- 

336 



THE STAR CLUSTERS 337 

ceived by the naked eye affects it hardly more than 
a star of the fifth or fourth to fifth magnitude, the 
minuteness of each star may be imagined." 

Others of these clusters are so remote that the 
separate stars are not distinguishable, especially at 
the center, and their distances are entirely beyond 
our present powers of direct measurement, although 
methods of estimating them are in "process of de- 
velopment. If gravitation is regnant among the 
uncounted components of stellar clusters, as doubt- 
less it is, these stars must be in rapid motion, al- 
though our photographs of measurements have been 
made too recently for us to detect even the slighest 
motion in any of the component , stars of a cluster. 
The only variations are changes of apparent magni- 
tude, of a type first detected in a large number of 
stars in Omega Centauri, by Bailey of Harvard, who 
by comparison of photographs of the globular clus- 
ters was the first to find variable stars quite nu- 
merous in these objects. Their unexplained varia- 
tions of magnitude take place with great rapidity, 
often within a few hours. 

There are about a hundred of these globular clus- 
ters, and the radial velocities of ten of them have 
been measured by Slipher and found to range from a 
recession of 410 to an approach of 225 kilometers 
per second. These excessive velocities are compar- 
able with those found for the spiral nebulae. Shapley 
has estimated the distances of many of these bodies, 
which contain a large number of variable stars of 
the Cepheid type. By assuming their absolute mag- 
nitudes equal to those of similar Cepheids at known 
distances, he finds their distance represented by the 
inconceivably minute parallax of 0".00012, corres- 
ponding to 30,000 light-years. This research also 



338 ASTRONOMY TO-DAY 

places the globular clusters far outside and inde- 
pendent of our Galactic system of stars. The dis- 
tribution of the globular clusters has also been in- 
vestigated, and these interesting objects are found 
almost exclusively in but one hemisphere of the sky. 
Its center lies in the rich star clouds of Scorpio and 
Sagittarius. Success in finding the distances of 
these objects has made it possible to form a general 
idea of their distribution in three-dimensional space. 

The numerous variable stars in any one cluster 
are remarkable for their uniformity. Accepting 
variables of this type as a constant standard of 
absolute brightness, and assuming that the differ- 
ences of average magnitude of the variables in dif- 
ferent clusters are entirely due to differences of 
distance, the relative distances of many clusters were 
ascertained with considerable accuracy. Then it was 
found that the average absolute magnitude of the 
twenty-five brighest stars in a cluster is also a uni- 
form standard, or about 1,3 magnitudes brighter 
than the mean magnitude of the variables. This 
new standard was employed in ascertaining the dis- 
tances of other clusters not containing many vari- 
ables. 

Shapley further shows that the linear dimensions 
of the clusters are nearly uniform, and the "proper 
relative positions in space are charted for sixty-nine 
of these objects. We can determine the scale of the 
charts, if we know the absolute brightness of our 
primary standard — ^the variable stars ; and this is de- 
duced from a knowledge of the distances of variables 
of the same type in our immediate stellar system. 

The most striking of all the globular clusters. 
Omega Centauri, comes out the nearest; neverthe- 
less it is distant 6.5 kiloparsecs. A kiloparsec is a 



THE STAR CLUSTERS 339 

thousand parsecs, and is the equivalent of 3,256 light- 
years. At the inconceivable distance of sixty-seven 
kiloparsecs, or more than 200,000 light-years, is the 
most remote of the globular clusters, known to as- 
tronomers as N.G.C. 7006, from its number in the 
catalogue which records its position in the sky, the 
New General Catalogue of nebulae by Dreyer of 
Armagh. 

The clusters are widely scattered, and their center 
of diffusion is about twenty kiloparsecs on the 
Galactic plane toward the region of Scorpio-Sagit- 
tarius. Marked symmetry with reference to this 
plane makes it evident that the entire system of 
globular clusters is associated with the Galaxy itself. 
But to conceive of this it is necessary to extend our 
ideas of the actual dimensions of the Galactic sys- 
tem. Almost on the circumference of the great 
system of globular clusters our local stellar system 
is found, and it contains probably all the naked-eye 
stars, with millions of fainter ones. Its size seems 
almost diminutive, only about one kiloparsec in 
diameter. The relative location of our local stellar 
system shows why the globular clusters appear to 
be crowded into one hemisphere only. 

Shapley suggests that globular clusters can exist 
only in empty space, and that when they enter the 
regions of space tenanted by stars, they dissolve into 
the well-known loose clusters and the star clouds of 
the Milky Way. Strangely the radial velocities of 
the clusters already observed show that most of them 
are traveling toward this region, and that some will 
enter the stellar regions within a period of the order 
of a hundred million years. 

The actual dimensions of globular clusters are 
not easy to determine, because the outer stars are 



340 ASTRONOMY TO-DAY 

much scattered. To a typical cluster, Messier 3, 
Shapley assigns a diameter of 150 parsecs, which 
makes it comparable with the size of the stellar clus- 
ter to which the sun belongs. Also on certain likely 
assumptions, he finds that the diameter of the great 
cluster in Hercules, the finest one in our northern 
sky, is about 350 parsecs, and its distance no less 
than 30,000 parsecs; in other words, the stagger- 
ing distance that light would require 9,750,000 years 
to travel over. While these distances can never be 
verified by direct measurement, it lends great weight 
to the three methods of indirect measurement, or 
estimation, (1) from the diameter of the image of 
the clusters, (2) from the mean magnitude of the 
twenty-five brightest stars, and (3) from the mean 
magnitude of the short period variables, that they 
are in excellent agreement. 



CHAPTER LV 

MOVING CLUSTERS 

RECENT researches on the proper motions of stars 
' have brought to light many groups of stars whose 
individual members have equal and parallel veloc- 
ities. Eddington calls these moving clusters. The 
component stars are not exceptionally near to each 
other, and it often happens that other stars not be- 
longing to the group are actually interspersed among 
them. They may be likened to double stars which 
are permanent neighbors, with some orbital motion, 
though exceedingly slow. 

The connection is rather one of origin ; occurring 
in the same region of space, perhaps, from a single 
nebula. They set out with the same motion, and 
have "shared all the accidents of the journey to- 
gether." Their equality of motion is intact because 
any possible deflections by the gravitative pull of the 
stellar system is the same for both. Mutual at- 
traction may tend to keep the stars together, but 
their community of motion persists chiefly because 
no forces tend to interfere with it. In this way 
physically connected pairs may be separated by very 
great distances. 

So with the moving clusters: their component 
stars may be widely separate on the celestial sphere, 
but equality of their motions affords a clue to their 
association in groups. The Hyades, a loose cluster 
in Taurus, is a group of thirty-nine stars, within 

341 



342 ASTRONOMY TO-DAY 

an area of about 15 degrees square, which has 
been pretty fully investigated, especially by the late 
Professor Lewis Boss; and no doubt many fainter 
stars in the same region will ultimately be found to 
belong to the same group. 

If we draw arrows on a chart representing the 
amount and direction of the proper motions of these 
stars, these arrows must all converge toward a point. 
This shows that their motions are parallel in space. 
It is a relatively compact group, and the close con- 
vergence shows that their individual velocities must 
agree within a small fraction of a kilometer per 
second. Radial velocity measures of six of the com- 
ponent stars are in very satisfactory accord, giving 
45.6 kilometers per second for the entire group. 

We can get the transverse velocity, and there- 
from the distances of the stars, which are among the 
best known in the heavens, because the proper 
motions are very accurately known. The mean 
parallax of the group by this indirect method comes 
out 0".025, agreeing almost exactly with the direct 
determination by photography, 0".023, by Kapteyn, 
De Sitter, and others. 

Eddington concludes that this Taurus group is a 
globular cluster with a slight central condensation. 
Its entire diameter is about ten parsecs, and its 
known motion enables us to trace its past and 
future history. It was nearest the sun 800,000 
years ago, when it was at about half its present 
distance. Boss calculated that in 65 million years, 
if the present motion is maintained, this group 
will have receded so far as to appear like an or- 
dinary globular cluster 20' in diameter, its stars 
ranging from the ninth to the twelfth apparent mag- 
nitude. We may infer that the motion will likely 



MOVING CLUSTERS 343 

continue undisturbed, because there are interspersed 
among the group many stars not belonging to it, and 
these have neither scattered its members nor sensi- 
bly interfered with the parallelism of their motion. 

Another moving cluster, the similarity of proper 
motion of whose component stars was first pointed 
out by Proctor, is known as the Ursa Major system, 
which embraces primarily Beta, Gamma, Delta, 
Epsilon, and Zeta Ursae Ma j oris, or five of the seven 
stars that mark the familiar Dipper. But as many 
as eight other stars widely scattered are thought to 
belong to the same system, including Sirius and 
Alpha Corona Borealis. The absolute motion 
amounts to 28.8 kilometers per second, and is ap- 
proximately parallel to the Galaxy. Turner has 
made a model of the cluster, which has the form of 
a flat disk. 

Among stars of the Orion type of spectrum are 
several examples of moving clusters. The Pleiades 
together with many fainter stars form another mov- 
ing cluster; as also do the brighter stars of Orion, 
together with the faint cloudlike extensions of the 
great nebula in Orion, whose radial velocity agrees 
with that of the stars in the constellation. Still 
another very remarkable moving cluster is in 
Perseus, first detected by Eddington, and embracing 
eighteen stars, the brightest of which is Alpha Persei. 

The further discovery of moving clusters is most 
important in the future development of stellar as- 
tronomy, because with their aid we can find out the 
relative distribution, luminosity, and distance of 
very remote stars. So far the stars found associated 
in groups are of early types of spectrum; but the 
Taurus cluster embraces several members equally 
advanced in evolution with the sun, and in the more 



344 ASTRONOMY TO-DAY 

scattered system of Ursse Major there are three 
stars of Type F. 

"Some of these systems," Eddington concludes, 
"would thus appear to have existed for a time com- 
parable with the lifetime of an average star. They 
are wandering through a part of space in which are 
scattered stars not belonging to their system — in- 
terlopers penetrating right among the cluster stars. 
Nevertheless, the equality of motion has not been 
seriously disturbed. It is scarcely possible to avoid 
the conclusion that the chance attractions of stars 
passing in the vicinity have no appreciable effect on 
stellar motions; and that if the motions change in 
course of time (as it appears they must do) this 
change is due, not to the passage of individual stars, 
but to the central attraction of the whole stellar uni- 
verse, which is sensibly constant over the volume of 
space occupied by a moving cluster." 



CHAPTER LVI 

THE TWO STAR STREAMS 

CONSIDER the ships on the Atlantic voyaging be- 
tween Europe and America: at any one time 
there may be a hundred or more, all bound either 
east or west, some moving in interpenetrating 
groups, individuals frequently passing each other, 
but rarely or never colliding. We might say, there 
are two great streams of ships, one moving east and 
the other west. 

Now in place of each ship, imagine a hundred 
ships, and magnify their distances from each other 
to the vast distances that the stars are from each 
other, and all in motion in two great streams as 
before. This will convey some idea of the rela- 
tively recent discovery, called by astronomers "star- 
streaming." 

Early in this century the investigation of mov- 
ing clusters began to reveal the fact that the motions 
of the stars were not at random throughout the 
universe, and about 1904 Kapteyn was the first to 
show that the stellar motions considered in great 
groups are very far from being haphazard, but that 
the stars tend to travel in two great streams, or 
favored directions. This was ascertained by analyz- 
ing the proper motions of stars in the sky, many 
thousands of them, and correcting all for the effect 
which the known motion of the sun would have upon 
them. The corrected motion, or part that is left 

345 



346 ASTRONOMY TO-DAY 

over, is known as the star's own motion, or motus 
peculiaris. 

This important investigation was very greatly- 
facilitated by the general catalogue of 6,188 stars 
well distributed over the entire sky, the work of the 
late Professor Boss. It was published by the Carne- 
gie Institution of Washington, and includes all stars 
down to the sixth magnitude. Boss was very criti- 
cal in the matter of stellar positions and proper 
motions and his work is the most accurate at present 
available. Excluding stars of the Orion type and 
the known members of moving clusters, Kapteyn's 
investigation was based on 5,322 stars, which he di- 
vided into seventeen regions of the sky, each north- 
ern region having an antipodal one in the southern 
hemisphere. 

Mathematical analysis of these regions showed 
them all in substantial agreement, with one excep- 
tion, and enabled Kapteyn to draw the conclusion 
that the stars of one stream, called Drift I, move 
with a speed of thirty-two kilometers per second, 
while those of the other. Drift II, travel with a speed 
of eighteen kilometers per second. Their directions 
are not, like those of east and west bound ships, 180 
degrees from each other, but are inclined at an 
angle of 100 degrees. Drift I embraces about three- 
fifths of the stars, and Drift II the remaining two- 
fifths. Quite as remarkable as the drifts themselves 
is the fact that the relative motion of the two is very 
closely parallel to the plane of the Milky Way. 

This epochal research has very great significance 
in all investigations of stellar motions, and it has 
been verified in various ways, particularly by the 
Astronomer Royal, Sir Frank Dyson, who limited the 
stars under consideration to 1,924 in number, but 



THE TWO STAR STREAMS 347 

all having very large proper motions. In this way 
the two streams are even more characteristically 
marked. But radial velocity determinations afford 
the ultimate and most satisfactory test, and Camp- 
bell has this investigation in hand, classifying the 
stars in their streaming according to the type. 

Type A stars are so far found to be confirmatory. 
Turning to the question of physical differences be- 
tween the stars of the two streams, Eddington in- 
quires into the average magnitude of the stars in 
both drifts, and their spectral type. Also whether 
they are distributed at the same distance from the 
sun, and in the same proportion in all parts of the 
sky. His conclusion is that there is no important 
difference in the magnitudes of the stars constitut- 
ing the two drifts. Regarding their spectra, stars 
of early and late types are found in both streams, 
with a somewhat higher proportion of late types 
among the stars of Drift II than those of Drift I. 
Campbell and Moore of the Lick Observatory have 
investigated seventy-three planetary nebulse which 
exhibit the phenomena of star-streaming, and have 
motions which are characteristic of the stars. 

Dealing with the very important question whether 
the two streams are actually intermingled in space, 
Eddington finds them nearly at the same mean dis- 
tance and thoroughly intermingled, and there is no 
possible hypothesis of Drifts I and II passing one 
behind the other in the same line of sight. A third 
drift, to which all the Orion stars belong, is under 
investigation, together with comprehensive analysis 
of the drifts according to the spectral type of all the 
stars included. 

The farther research on star-streaming is pushed, 
the more it becomes evident that a third stream, 



348 ASTRONOMY TO-DAY 

called Drift 0, is necessary, especially to include B- 
ty'pe stars. The farther we recede from the sun, the 
more this drift is in evidence. At the average dis- 
tances of B-type stars, the observed motions are al- 
most completely represented by Drift alone. Halm 
of Cape Town concludes from recent investigations 
that the double-drift phenomena (Drifts I and II) 
is of a distinctly local character, and concerns chiefly 
the stars in the vicinity of the solar system; while 
stars at the greatest distances from the sun belong 
preeminently to Drift 0. 

The 60-inch reflector on Mount Wilson gathers 
sufficient light so that the spectra of very faint stars 
can be photographed, and a discussion of velocities 
derived in this manner has shown that Kapteyn's 
two star streams extend into space much farther 
than it was possible to trace them with the nearer 
stars. Star-streaming, then, may be a phenomenon 
of the widest significance in reference to the entire 
universe. 

As to the fundamental causes for the two opposite 
and nearly equal star streams, it is early perhaps 
to even theorize upon the subject. Eddington, how- 
ever, finds a possible explanation in the spiral 
nebulae, which are so numerous as to indicate the 
certainty of an almost universal law compelling 
matter to flow in these forms. Why it does so, we 
cannot be said to know; but obviously matter is 
either flowing into the nucleus from the branches of 
the spiral, or it is flowing out from the nucleus into 
the branches. Which of the two directions does not 
matter, because in either case there would be cur- 
rents of matter in opposite directions at the points 
where the arms merge in the central aggregation. 
The currents continue through the center, because 



THE TWO STAR STREAMS 349 

the stars do not interfere with one another's paths. 
As Eddington concludes : "There then we have an 
explanation of the prevalence of motions to and fro 
in a particular straight line ; it is the line from which 
the spiral branches start out. The two star streams 
and the double-branched spirals arise from the 
same cause." 



CHAPTER LVII 

THE GALAXY OR MILKY WAY 

GRANDEST of all the problems that have occu- 
pied the mind of man is the distribution of the 
stars throughout space. To the earliest astron- 
omers who knew nothing about the distances of the 
stars, it was not much of a problem because they 
thought all the fixed stars were attached to a re- 
volving sphere, and therefore all at essentially the 
same distance ; a very moderate distance, too. Even 
Kepler held the idea that the distances of indi- 
vidual stars from each other are much less than 
their distances from our sun. 

Thomas Wright, of Durham, England, seems to 
have been the first to suggest the modern theory of 
the structure of the stellar universe, about the 
middle of the eighteenth century. His idea was 
taken up by Kant who elaborated it more fully. It 
is founded on the Galaxy, the basal plane of stellar 
distribution, just as the ecliptic is the fundamental 
circle of reference in the solar system. 

What is the Galaxy or Milky Way? 

Here is a great poet's view of the most poetic 
object in all nature: 

A broad and ample road, whose dust is gold, 
And pavement stars, as stars to thee appear 
Seen in the Galaxy, that Milky Way 
Which nightly as a circling zone thou seest 
Powder'd with stars. 

Milton, P. L. vii, 580. 
350 



THE GALAXY OR MILKY WAY 351 

Were the earth transparent as crystal, so that we 
could see downward through it and outward in all 
directions to the celestial sphere, the Galaxy or 
Milky Way would appear as a belt or zone of 
cloudlike luminosity extending all the way round 
the heavens. As the horizon cuts the celestial 
sphere in two, we see at any one time only one-half 
of the Milky Way, spanning the dome of the sky as 
a cloudlike arch. 

As the general plane of the Galaxy makes a large 
angle with our equator, the Milky Way is continu- 
ally changing its angle with the horizon, so that it 
rises at different elevations. One-half of the 
Milky Way will always be below our horizon, and 
a small region of it lies so near the south pole of 
the heavens that it can never be seen from medium 
northern latitudes. 

Galileo was the first to explain the fundamental 
mystery of this belt, when he turned his telescope 
upon it and found that it was not a continuous 
sheet of faint light, as it seemed to be, but was 
made up of countless numbers of stars, individually 
too faint to be visible to the naked eye, but whose 
vast number, taken in the aggregate, gave the well- 
known effect which we see in the sky. In some 
regions, as Perseus, the stars are more numerous 
than in others, and they are gathered in close 
clusters. The larger the telescope we employ, the 
greater the number of stars that are seen as we 
approach the Galaxy on either side ; and the farther 
we recede from the Galaxy and approach either of 
its poles fewer and fewer stars are found. Indeed, 
if all the stars visible in a 12-inch telescope could 
be conceived as blotted out, nearly all the stars that 
are left would be found in the Galaxy itself. 



352 ASTRONOMY TO-DAY 

The naked eye readily notes the variations in 
breadth and brightness of the galactic zone. 
Nearly a third of it, from Scorpio to Cygnus, is 
split into two divisions nearly parallel. In many 
regions its light is interrupted, especially in Cen- 
taurus, vi^here a dark starless region exists, known 
as the "coal sack." Sir John Herschel, who fol- 
lowed up the stellar researches of his father, Sir 
William, in great detail, places the north pole of 
the Galactic plane in declination 37 degrees N., and 
right ascension 12 h. 47 m. This makes the plane 
of the Milky Way lie at an angle of about 60 de- 
grees with the ecliptic, which it intersects not far 
from the solstices. 

Now Kant, in view of the two great facts about 
the Galaxy known in his time, (1) that it wholly 
encircles the heavens, and (2) that it is composed 
of countless stars too faint to be individually visible 
to the naked eye, drew the safe conclusions that 
the system of the stars must extend much farther 
in the direction of the Milky Way than in other 
directions. 

This theory of Kant was next investigated from 
an observational standpoint by Sir William Her- 
schel, the ultimate goal of whose researches was 
always a knowledge of the construction of the 
heavens. The present conclusion is that we may 
regard the stellar bodies of the sidereal universe 
as scattered, without much regard to uniformity, 
throughout a vast space having in general the shape 
of a thick watch, its thickness being perhaps one- 
tenth its diameter. On both sides of this disk of 
stars, and clustered about the poles of the sidereal 
system are the regions occupied by vast numbers 
of nebulae. The entire visible universe, then, would 



^B» *Mr 




■Kkw 









The Great Nebula of Andromeda^ Largest (Apparently) of all the 

Spiral Nebulae. This nebula can be seen very faintly with the naked 

eye, but no telescope has yet resolved it into separate stars. {Flioto, 

Yerkes Observatory.) 



THE GALAXY OR MILKY WAY 353 

be spheroidal in general shape. The plane of the 
Milky Way passes through the middle of this aggre- 
gation of stars and nebulae, and the solar system is 
near the center of the Milky Way. Throughout the 
watch-form space the stars are clustered irregu- 
larly, in varied and sometimes fantastic forms, but 
without approach to order or system. If we except 
some of the star groups and star clusters and con- 
sider only the naked-eye stars, we find them scat- 
tered with fair approach to uniformity. 

The watch-shaped disk is not to be understood as 
representing the actual form of the stellar system, 
but only in general the limits within which it is 
for the most part contained. 

A vigorous attack on the problem of the evolu- 
tion and structure of the stellar universe as a whole 
is now being conducted by cooperation of many ob- 
servatories in both hemispheres. It is known as 
the Kapteyn "Plan of Selected Areas," embracing 
206 regions which are distributed regularly over 
the entire sky. Besides this a special plan includes 
forty-six additional regions, either very rich or 
extremely poor in stars, or to which other inter- 
est attaches. 

Of all investigators Kapteyn has gone into the 
question of our precise location in the Milky Way 
most thoroughly, concluding that the solar system 
lies, not at the center in the exact plane, but some- 
what to the north of the Galaxy. Discussing the 
Sirian stars he finds that if stars of equal bright- 
ness are compared, the Sirians average nearly three 
times more distance from the sun than those of 
the solar type. So, probably, the Sirians far ex- 
ceed the Solars in intrinsic brightness. Farther, 
Kapteyn concludes that the Galaxy has no connec- 

Sci. Vol. 2—12 



354 ASTRONOMY TO-DAY 

tion with our solar system, and is composed of a 
vast encircling annulus or ring of stars, far ex- 
ceeding in number the stars of the great central 
solar cluster, and everywhere exceedingly remote 
from these stars, as well as differing from them in 
physical type and constitution, go it would be 
mainly the mere element of distance that makes 
them appear so faint and crowded thickly together 
into that gauzy girdle which we call the Galaxy. 

The Milky Way reveals irregularities of stellar 
density and star clustering on a large scale, with 
deep rifts between great clouds of stars. Modern 
photographs, particularly those of Barnard in Sagit- 
tarius, make this very apparent. Within the Milky 
Way, nearly in its plane and almost central, is what 
Eddington terms the inner stellar system, near the 
center of which is the sun. Surrounding it and 
near its plane are the masses of star clouds which 
make up the Milky Way. Whether these star 
clouds are isolated from the inner system or con- 
tinuous with it, is not yet ascertained. 

The vast masses of the Milky Way stars are very 
faint, and we know nothing yet as to their proper 
motions, their radial motions, or their spectra. 
Probably a few stars as bright as the sixth magni- 
tude are actually located in the midst of the Milky 
Way clusters, the fainter ninth magnitude stars 
certainly begin the Milky Way proper, while the 
stars of the twelfth or thirteenth magnitude carry 
us into the very depths of the Galaxy. 

It is now pretty generally believed that many of 
the dark regions of the Milky Way are due not to 
actual absence of stars so much as to the absorp- 
tion of light by intervening tracts of nebulous mat- 
ter on the hither side of the Galactic aggregations 



THE GALAXY OR MILKY WAY 355 

and, probably in fact, within the oblate inner stellar 
system itself. Easton has made many hundred 
counts of stars in galactic regions of Cygnus and 
Aquila where the range of intensity of the light is 
very marked ; in fact, the star density of the bright 
patches of the Galaxy is so far in excess of the 
density adjacent and just outside the Milky Way, 
that the conclusion is inevitable that this excess is 
due to the star clouds. 

Of the distance of the Milky Way we have very 
little knowledge. It is certainly not less than 1,000 
parsecs, and more likely 5,000 parsecs, a distance 
over which light would travel in about 16,000 years. 
Quite certainly all parts of the Galaxy are not at 
the same distance, and probably there are branches 
in some regions that lie behind one another. While 
the general regions of the nebulas are remote from 
the Galactic plane, the large irregular nebulse, as 
the Trifid, the Keyhole, and the Omega nebulse, are 
found chiefly in the Milky Way. 

In addition to the irregular nebulse many types 
of stellar objects appear to be strongly condensed 
toward the Milky Way, but this may be due to the 
inner stellar system, rather than a real relation to 
Galactic formations. Quite different are the Magel- 
lanic clouds, which contain many gaseous nebulse and 
are unique objects of the sky, having no resemblance 
to the true spiral nebulse which, as a rule, avoid the 
Galactic regions. Worthy of no^'^ also is the theory 
of Easton that the Milky Way has itself the form 
of a double-branched spiral, which explains the 
visible features quite well, but is incapable of either 
disproof or verification. The central nucleus he 
locates in the rich Galactic region of Cygnus, with 
the sun well outside the nucleus itself. By combin- 



356 ASTRONOMY TO-DAY 

ing the available photographs of the Galaxy, he has 
produced a chart which indicates in a general way 
how the stellar aggregations might all be arrayed 
so as to give the effect of the Galaxy as we see it. 

Shapley, at Mount Wilson, has studied the struc- 
ture of the Galactic system, in which he has been 
aided by Mrs. Shapley. An interesting part of this 
work relates to the distribution of the spiral 
nebulae, and to certain properties of their sys- 
tematic recessional motion, suggesting that the en- 
tire Galactic system may be rapidly moving through 
space. Apparently the spiral nebulse are not distant 
stellar organizations or "island universes," but truly 
nebular structures of vast volume which in general 
are actively repelled from stellar systems. A tenta- 
tive cosmogonic hypothesis has been formulated to 
accounlj for the motions', distribution, and observed 
structure of clusters and spiral nebulse. 

An additional great problem of the Galaxy is a 
purely dynamical one. Doubtless it is in some sort 
of equilibrium, according to Eddington, that is to 
say, the individual stars do not oscillate to and fro 
across the stellar system in a period of 300 million 
years, but remain concentrated in clusters as at 
present. Poincare has considered the entire Milky 
Way as in stately rotation, and on the assumption 
that the total mass of the inner stellar system is 
1,000,000,000 times the sun's mass, and that the dis- 
tance of the Milky Way is 2,000 parsecs, the angu- 
lar velocity for equilibrium comes out 0".5 per cen- 
tury. That is to say, a complete revolution would 
take place in about 250 million years. 



CHAPTER LVIII 

STAR CLOUDS AND NEBULA 

FROM star clusters to nebulae, only a century ago, 
the transition was thought to be easy and imme- 
diate. Accuracy in determining the distances of 
stars was just beginning to be reached, the clusters 
were obviously of all degrees of closeness following 
to the verge of irresolvability, and it was but natural 
to jump to the conclusion that the mystery of the 
nebulae consisted in nothing but their vaster dis- 
tance than that of clusters, and it was believed that 
all nebulae would prove resolvable into stars when- 
ever telescopes of sufficiently great power could be 
constructed. 

But the development of the spectroscope soon 
showed the error of this hypothesis, by revealing 
bright lines in the nebular spectra showing that 
many nebulae emit light that comes from glowing in- 
candescent gas, not from an infinitude of small stars. 

In pre-telescope days nothing was known about 
the nebulae. The great nebula in Andromeda, and 
possibly the great nebula in Orion, are alone visible 
to the naked eye, but as thus seen they are the 
merest wisps of light, the same as the larger clus- 
ters are. Galileo, Huygens and other early users of 
the telescope made observations of nebulae, but long- 
focus telescopes were not well adapted to this work. 
Simon Mayer has left us the first drawing of a 
nebula, the Orion nebula as he saw it in 1612. The 

357 



858 ASTRONOMY TO-DAY 

vast light-gathering power of the reflectors built by 
Sir William Herschel first afforded glimpses of the 
structure of the nebulse, and if his drawings are 
critically compared with modern ones, no case of 
motion with reference to the stars or of change in 
the filaments of the nebulse themselves has been 
satisfactorily made out. 

Only very recently has the distance of a nebula 
been determined, and the few that have been meas- 
ured seem to indicate that the nebulse are at dis- 
tances comparable with the stars. Of all celestial 
objects the nebulse fill the greatest angles, so that 
we are forced to conclude, with regard to the actual 
size of the greater nebulse as they exist in space, 
that they far surpass all other objects in bulk. 

Photography invaded the realm of the nebulse 
in 1880, when Dr. Henry Draper secured the first 
photograph of the nebula of Orion. Theoretically 
photography ought to help greatly in the study of 
the nebulse, and enable us in the lapse of centuries 
to ascertain the exact nature of the changes which 
must be going on. The differences of photographic 
processes, of plates, of exposure and development 
produce in the finished photograph vastly greater 
differences than any actual changes that might be 
going on, so that we must rely rather on optical 
drawings made with the telescope, or on drawings 
made by expert artists from photographs with many 
lengths of exposure on the same object. 

The great work on nebulse and star clusters re- 
cently concluded by Bigourdan of the Paris Observa- 
tory and published in five volumes received the 
award of the gold medal of the Royal Astronomical 
Society, While D'Arrest measured about 2,000 
nebulse, and Sir John Herschel about double that 



STAR CLOUDS AND NEBULA 359 

number in both hemispheres, Bigourdan has meas- 
ured about 7,000. His work forms an invaluable 
lexicon of information concerning the nebulae. 

Classification of the nebulae is not very satis- 
factory, if made by their shapes alone. There are 
perhaps fifteen thousand nebulae in all that have 
been catalogued, described, and photographed. 
Dreyer's new general catalogue (N.G.C.) is the best 
and most useful. Many of the nebulae, especially 
the large ones, can only be classified as irregular 
nebulae. The Orion nebula is the principal one of 
this class, revealing an enormous amount of com- 
plicated detail, with exceptional brilliancy of many 
regions and filaments. An extraordinary multiple 
star, Theta Orionis, occupies a very prominent posi- 
tion in the nebula, and photographs by Pickering 
have brought to light curved filaments, very faint 
and optically invisible, in the outlying regions which 
give the Orion nebula in part a spiral character. 
But the delicate optical wisps of this nebula are well 
seen, even in very small telescopes. Its spectrum 
yields hydrogen, helium and nitrogen. The Orion 
nebula is receding from the earth about eleven miles 
in every second. Keeler and Campbell have shown 
that nearly every line of the nebular spectrum is a 
counterpart of a prominent dark line in the spec- 
trum of the brighter stars of the constellation of 
Orion. A recent investigator of the distribution of 
luminosity in the great nebula of Orion finds that 
radiations from nebulium are confined chiefly to the 
Huygenian region of the nebula and its immediate 
neighborhood. 

Photography has revealed another extraordinary 
nebula or group of nebulae surrounding the stars 
in the Pleiades, which the deft manipulation of 



360 ASTRONOMY TO-DAY 

Barnard has brought to light. All the stars and the 
nebula are so interrelated that they are obviously 
bound together physically, as the common proper 
motion of the stars also appears to show. Also in 
the constellation Cygnus, Barnard has discovered 
very extensive nebulosities of a delicate filmy cloud- 
like nature which are wholly invisible with tele- 
scopes, but very obvious on highly sensitive plates 
with long exposures. 

Another class of these objects are the annular and 
elliptic nebulse which are not very abundant. The 
southern constellation Grus, the crane, contains a 
fine one, but by far the best example is in the con- 
stellation Lyra. It is a nearly perfect ring, elliptic 
in figure, exceedingly faint in small telescopes ; but 
large instruments reveal many stars within the an- 
nulus, one near the center which, although very faint 
to the eye, is always an easy object on the -photo- 
graphic plate, because it is rich in blue and violet 
rays. The parallax of the ring nebula in Lyra comes 
out only one-sixth of that of the planetary nebulae, 
and the least greatest diameters of this huge con- 
tinuous ring are 250 and 330 times the orbit of 
Neptune. 

Planetary nebulae and nebulous stars are yet 
another class of nebulae, for the most part faint and 
small, resembling in some measure a planetary disk 
or a star with nebulous outline. Practically all are 
gaseous in composition, and have large radial veloci- 
ties. Probably they are located within our own stel- 
lar system. The parallaxes of several of them have 
been measured by VanMaanen: one of the very small 
angle 0".023, which enables us to calculate the diam- 
eter of this faint but interesting object as equal to 
nineteen times the orbit of Neptune. 



CHAPTER LIX 

THE SPIRAL NEBULA 

1AST and most important of all are the spiral 
i nebulse. The finest example is in the constellation 
Ganes Venatici, and its spiral configuration was 
first noted by Lord Rosse, an epoch-making dis- 
covery. The convolutions of its spiral are filled with 
numerous starlike condensations, themselves en- 
gulfed in nebulosity. Photography possesses a vast 
advantage over the eye in revealing the marvelous 
character of this object, an inconceivably vast celes- 
tial whirlpool. Naturally the central regions of the 
whorl would revolve most swiftly, but no comparison 
of drawings and photographs, separated by intervals 
of many years, has yet revealed even a trace of any 
such motion. 

The number of large spiral nebulse is not very 
great; the largest of all is the great nebula of An- 
dromeda, whose length stretches over an arc of seven 
times the breadth of the moon, and its width about 
half as great. This nebula is a naked-eye object near 
Eta Andromedse, and it is often mistaken for a 
comet. Optically it was always a puzzle, but photo- 
graphs by Roberts of England first revealed the true 
spiral, with ringlike formations partially distinct, 
and knots of condensing nebulosity as of companion 
stars in the making. While its spectrum shows the 
nongaseous constitution of this nebula, no telescope 
has yet resolved it into component stars, 

361 



362 ASTRONOMY TO-DAY 

Systematic search for spiral nebulae by Keeler, 
and later continued by Perrine, at the Lick Observ- 
atory, with the 36-inch Crossley reflector, dis- 
closed the existence of vast numbers of these objects, 
in fact many hundreds of thousands by estimation ; 
so that, next to the stars, the spiral nebulse are by 
far the most abundant of all objects in the sky. They 
present every phase according to the angle of their 
plane with the line of sight, and the convolutions 
of the open ones are very perfectly marked. Many 
are filled with stars in all degrees of condensation, 
and the appearance is strongly as if stars are here 
caught in every step of the process of making. 

The vast multitude of the spiral nebulse indicates 
clearly their importance in the theory of the cos- 
mogony, or science of the development of the ma- 
terial universe. Curtis of the Lick Observatory has 
lately extended the estimated number of these ob- 
jects to 700,000. He has also photographed with the 
Crossley reflector many nebulse with lanes or dark 
streaks crossing them longitudinally through or 
near the center. These remarkable streaks appear 
as if due to opaque matter between us and the lumi- 
nous matter of the nebula beyond. Perhaps a dark 
ring of absorptive or occulting matter encircles the 
nebula in nearly the same plane with the luminous 
whorls. Duncan has employed the 60-inch Mount 
Wilson reflector in photographing bright nebulse 
and star clusters in the very interesting regions of 
Sagittarius. One of these shows unmistakable dark 
rifts or lanes in all parts of the nebula, resembling 
the dark regions of the neighboring Milky Way. 

Pease of Mount Wilson has recently employed the 
60-inch and the 100-inch reflectors of the Mount 
Wilson Observatory to good advantage in photo- 



THE SPIRAL NEBULA 363 

graphing several hundred of the fainter nebulse. 
Many of these are spirals, and others present very 
intricate and irregular forms. A search was made 
for additional spirals among the smaller nebula 
along the Galaxy, but without success. Several of 
the supposedly variable nebulse are found to be un- 
changing. Many nights in each month when the 
moon is absent are devoted to a systematic survey 
of the smaller nebulse and their spectra by photog- 
raphy. The visible spiral figure of all these ob- 
jects is a double-branched curve, its two arms join- 
ing on the nucleus in opposing points, and coiling 
round in the same geometrical direction. The spiral 
nebulse, as to their distribution, are remote from 
the Galaxy, and the north Galactic polar region con- 
tains a greater aggregation than the south. The 
distances of the spiral nebulse are exceedingly great. 
They lie far beyond the planetary and irregular 
gaseous nebulae, like that of Orion, which are closely 
related to the stars forming part of our own 
system. Possibly the spiral nebulse are exterior or 
separate ''island universes." If so, they must be in- 
conceivably vast in size, and would develop, not into 
solar systems, but into stellar clusters. The enormous 
radial velocities of the spiral nebulse, averaging 800 
to 400 kilometers per second, or twentyfold that of 
the stars, tend to sustain the view that they may be 
''island universes," each comparable in extent with 
the universe of stars to which our sun belongs. 

Recent spectroscopic observations of the nebulse 
applying the principle of Doppler have revealed high 
velocities of rotation. Slipher of the Lowell Observa- 
tory made the first discovery of this sort and Van 
Maanen of Mount Wilson has detected in the great 
Ursa Major spiral. No. 101 in Messier's catalogue, 



364 ASTRONOMY TO-DAY 

a speed of rotation at five minutes of arc from the 
center that would correspond to a complete period 
in 85,000 years. As was to be expected, the nebula 
does not rotate as a rigid body, but the nearer the 
center the greater the angular velocity, and Van 
Maanen finds evidence of motion along the arms and 
away from the center. 

These great velocities appear to belong to the 
spiral nebulse as a class, and not to other nebulae. 
Thirteen nebulas investigated by Keeler are as a 
whole almost at rest relatively to our system, as are 
the large irregular objects in Orion, and the Trifid 
nebula. This would seem to indicate that the spiral 
nebulae form systems outside our own and inde- 
pendent of it. 

Quite different from the spirals in their distribu- 
tion through space are the planetary nebulae. The 
spirals follow the early general law of nebulae ar- 
rangement, that is, they are concentrated toward 
the poles of the Galaxy; but the planetary nebulae, 
on the other hand, are very few near the poles and 
show a marked frequency toward the Galactic plane« 
Campbell and Moore have found spectroscopic evi- 
dence of internal rotatory motion in a large propor- 
tion of the planetary nebulae. 

The distribution of the nebulae throughout space, 
like that of the stars, is still under critical investiga- 
tion, but the location of vast numbers of the more 
compact nebulae on the celestial sphere is very ex- 
traordinary. The Milky Way appears to be the 
determining plane in both cases; the nearer we 
approach it the more numerous the stars become, 
whereas this is the general region of fewest nebulae 
and they increase in number outward in both direc- 
tions from the Galaxy, and toward both poles of the 



THE SPIRAL NEBULA 365 

Galactic circle. Obviously this relation, or contra- 
relation of stars and nebulae on such a vast scale is 
not accidental, and it also must be duly accounted 
for in the true theory of the cosmogony. The 
nebulae which are found principally in and near the 
Milky Way are the large irregular nebulae, and vast 
nebulous backgrounds, like those photographed by 
Barnard in Scorpio, Taurus and elsewhere, as well 
as the Keyhole, Omega, and Trifid nebulae. Allied 
to these backgrounds are doubtless some of the dark 
Galactic spaces, radiating little or no intrinsic light, 
and absorbing the light of the fainter stars beyond 
them. A peculiar veiled or tinted appearance has 
been remarked in some cases visually, and examina- 
tion of the photographs strongly confirms the ex- 
istence of absorbing nebulosity. 

The spiral nebulae are so abundant, and so much 
attention is now being given to them, both by ob-; 
servers and mathematicians, that their precise rela- 
tion to the stellar systems must soon be known ; that 
is, whether they are comparatively small objects 
belonging to the stellar system, or independent 
systems on the borders of the stellar system, or as 
seems more likely, vast and exceedingly remote 
galaxies comparable with that of the Milky Way 
itself. Our knowledge of the motions of the spirals, 
both radial and angular, is increasing rapidly, and 
must soon permit accurate general conclusions to 
be drawn. 



CHAPTER LX 

COSMOGONY 

DOWN to the middle of the last century and later, 
it was commonly believed that in the begin- 
ning the cosmos came into being by divine fiat 
substantially as it is. Previously the earth had 
been "without form and void/' as in the Scrip- 
ture. Had it not been for the growth and gradual 
acceptance of the doctrine of evolution, and 
its reactionary effect upon human thought, it is 
conceivable that the early view might have persisted 
to the present day; but now it is universally held 
that everything in the heavens above and the earth 
beneath is subject more or less to secular change, 
and is the result of an orderly development through- 
out indefinite past ages, a progressive evolution 
which will continue through indefinite aeons of the 
future. 

In the writings of the Greek philosophers, and 
down through the Middle Ages we find the idea of 
an original "chaos" prevailing, with no indication 
whatever of the modern view of the process by which 
the cosmos came to be what they saw it and as it is 
to-day. If we go still farther back, there is no glim- 
mer of any ideas that will bear investigation by 
scientific method, however interesting they may be 
as purely philosophical conceptions. Many ancient 
philosophers, among them Anaxagoras, Democritus, 
and Anaximenes, regarded the earth as the product 

366 



COSMOGONY 867 

of diffused matter in a state of the original chaos 
having fallen together haphazard, and they even 
presumed to predict its future career and ultimate 
destiny. 

In Anaximander and Anaximenes alone do we find 
any conception of possible progress; their thought 
was that as the world had taken time to become what 
it is, so in time it would pass, and as the entire uni- 
verse had undergone alternate renewal and destruc- 
tion in the past, that would be its history in the 
future. Aristotle, Ptolemy, and others appear to 
have held the curious notion that although every- 
thing terrestrial is evanescent, nevertheless the 
cosmos beyond the orbit of the moon is imperishable 
and eternal. 

By tracing the history of the intellectual develop- 
ment of Europe we may find why it was that scien- 
tific speculation on the cosmogony was delayed until 
the 18th century, and then undertaken quite inde- 
pendently by three philosophers in three different 
countries. Swedenborg, the theologian, set down in 
due form many of the principles that underlie the 
modern nebular hypothesis. Thomas Wright of 
Durham whose early theory of the arrangement of 
stars in the Galaxy we have already mentioned, 
speculated also on the origin and development of 
the universe, and his writings were known to Kant, 
who is now regarded as the author of the modern 
nebular hypothesis. This presents a definite me- 
chanical explanation of the development and forma- 
tion of the heavenly bodies, and in particular those 
composing the solar system. 

Kant was illustrious as a metaphysician, but he 
was a great physicist or natural philosopher as well, 
and he set down his ideas regarding the cosmogony 



368 ASTRONOMY TO-DAl 

with precision. Learned in the philosophy of the 
ancients, he did not follow their speculative concep- 
tions, but merely assumed that all the materials 
from which the bodies of the solar system have been 
fashioned were resolved into their original ele- 
ments at the beginning, and filled all that part of 
space in which they now move. True, this is pretty 
near the chaos of the Greeks, but Kant knew of the 
operation of the Nev^tonian law of gravitation, 
which the Greeks did not. 

As a natural result of gravitative processes, Kant 
inferred that the denser portions of the original 
mass would draw upon themselves the less dense 
portions, whirling motions would be everywhere 
set up, and the process would continue until many 
spherical bodies, each with a gaseous exterior in 
process of condensation, had taken the place of the 
original elements which filled space. In this man- 
ner Kant would explain the sameness in direction 
of motion, both orbital and axial, of all the planets 
and satellites of our system. But many philos- 
ophers are of the opinion that Kant's hypothesis 
would result, not in the formation of such a collec- 
tion of bodies as the solar system is, but rather in a 
single central sun formed by common gravitation 
toward a single center. 

From quite another viewpoint the work of the 
elder Herschel is important here. No one knew the 
nebulse from actual observation better than he did ; 
but, while his ideas about their composition were 
wrong, he nevertheless conceived of them as gradu- 
ally condensing into stars or clusters of stars. And 
it was this speculative aspect of the nebulae, not as 
a possible means of accounting for the birth and 
development of the solar system, which constitutes 



COSMOGONY 369 

Herschers chief contribution to the nebular hypoth- 
esis. Classifying the nebulae which he had care- 
fully studied with his great telescopes, it seemed 
obvious to him that they were actually in all the 
different stages of condensation, and subsequent 
research has strongly tended to substantiate the 
Herschelian view. 

Then came Laplace, who took up the great hypoth- 
esis where Kant and Herschel had left it, added 
new and important conceptions in the light of his 
mature labors as mathematician and astronomer, 
and put the theory in definitive form, such that it 
has ever since been known under the name of La- 
placian nebular hypothesis. For reasons like those 
that prevailed with Kant, he began the evolution of 
the solar system with the sun already formed as 
the center, but surrounded by a vast incandescent 
atmosphere that filled all the space which the sun's 
family of planets now occupy. This entire mass, 
sun, atmosphere, and all, he conceived to have a 
stately rotation about its axis. With rotation of 
the mass and slow reduction of temperature in its 
outer regions, there would be contraction toward 
the solar center, and an increase in velocity of rota- 
tion until the whole mass had been much reduced 
in diameter at its poles and proportionately ex- 
panded at its equator. 

When the centrifugal force of the outer equa- 
torial masses finally became equal to the gravita- 
tional forces of the central mass, then these con- 
joined outer portions would be left behind as a 
ring, still revolving at the velocity it had acquired 
when detached. The revolution of the entire inner 
mass goes on, its velocity accelerating until a sim- 
ilar equilibration of forces is again reached, when 



370 ASTRONOMY TO-DAY 

a second rotating ring is left behind. Laplace con- 
ceived the process as repeated until as many rings 
had been detached as there are individual planets, 
all central about the sun, or nearly so. 

In all, then, we should have nine gaseous rings; 
the outer ones preceding the inner in formation, 
but not all existing as rings at the same time. Radi- 
ation from the ring on all sides would lead to rapid 
contraction of its mass, so that many nuclei of con- 
densation would form, of various sizes, all revolv- 
ing round the central sun in practically the same 
period. Laplace conceived the evolution of the ring 
to proceed still farther till the largest aggregation 
in it had drawn to itself all the other separate 
nuclei in the ring. 

This, then, was the planet in embryo, in effect a 
diminutive sun, a secondary incandescent mass en- 
dowed with axial rotation in the same direction as 
the parent nebula. With reduction of temperature 
by radiation, polar contraction and equatorial ex- 
pansion go on, and planetary rings are detached 
from this secondary mass in exactly the same way 
as from the original sun nebula. And these planet- 
ary rings are, in the Laplacian hypothesis, the em- 
bryo moons or planetary satellites, all revolving 
round their several planets in the same direction 
that the planets revolve about the sun. 

In the case of one of the planetary rings, its 
formation was so nearly homogeneous throughout 
that no aggregation into a single satellite was pos- 
sible ; all portions of the ring being of equal density, 
there was no denser region to attract the less dense 
regions, and in this manner the rings of Saturn 
were formed, in lieu of condensation into a sepa- 
rate satellite. Similarly in the case of the primal 



COSMOGONY 371 

solar ring that was detached next after the Jovian 
ring; there was such a nice balancing of masses 
and densities that, instead of a single major planet, 
we have the well-known asteroidal ring, composed 
of innumerable discrete minor planets. 

This, then, in bare outline, is the Laplacian nebu- 
lar hypothesis, and it accounted very well for the 
solar system as known in his day; the fairly regu- 
lar progression of planetary distances; their orbits 
round the sun all nearly circular and approximately 
in a single plane ; the planetary and satellite revolu- 
tions in orbit all in the same direction; the axial 
rotations of planets in the same direction as their 
orbital revolutions ; and the plane of orbital revolu- 
tion of the satellites practically coinciding with the 
plane of the planet's axial rotation. But the prin- 
ciple of conservation of energy was, of course, un- 
known to Laplace, nor had the mechanical equiv- 
alence of heat with other forms of energy been 
established in his day. 

In 1870, Lane of Washington first demonstrated 
the remarkable law that a gaseous sphere, in 
process of losing heat by radiation and contraction 
because of its own gravity, actually grows hotter 
instead of cooler, as long as it continues to be gas- 
eous, and not liquid or solid. So there is no need of 
postulating with Laplace an excessively high tem- 
perature of the original nebula. The chief objec- 
tion to Laplace's hypothesis by modern theorists is 
that the detachment of rings, though possible, 
would likely be a rare occurrence ; protuberances or 
lumps on the equatorial exterior of a swiftly revolv- 
ing mass would be more likely, and it is much 
easier to see how such masses would ultimately be- 
come planets than it is to follow the disruption of 



372 ASTRONOMY TO-DAY 

a possible ring and the necessary steps of the 
process by which it would condense into a final 
planet. The continued progess of research in many 
departments of astronomy has had important bear- 
ing on the nebular hypothesis, and we may rest 
assured that this hypothesis in somewhat modified 
form can hardly fail of ultimate acceptance, though 
not in every essential as its great originator left it. 

Lord Rosse's discovery of spiral nebulae, followed 
up by Keeler's photographic search for these bodies, 
revealing their actual existence in the heavens by 
the hundreds of thousands, has led to another 
criticism of the Laplacian theory. Could Laplace 
have known of the existence of these objects 
in such vast numbers, his hypothesis would no 
doubt have been suitably modified to account for 
their formation and development. It is generally 
considered that the ring of Saturn suggested to 
Laplace the ring feature in his scheme of origin of 
planets and satellites ; so far as we know, the Satur- 
nian ring is unique, the only object of its kind in the 
heavens. Whereas, next to the star itself, the spiral 
nebula is the type object which occurs most fre- 
quently. A theory, therefore, which will satisfac- 
torily account for the origin and development of 
spiral nebulae must command recognition as of great 
importance in the cosmogony. 

Such a theory has been set forth by Chamberlin 
and Moulton in their planetesimal hypothesis, ac- 
cording to which the genesis of spiral nebulae 
happens when two giant suns approach each other 
so closely that tide-producing effects take place on a 
vast scale. These suns need not be luminous; they 
may perhaps belong to the class of dark or extin- 
guished suns. The evidences of the existence of 



COSMOGONY 373 

such in vast numbers throughout the universe is 
thought to be well established. 

Now, on close approach, what happens? There 
will be huge tides, and the nearer the bodies come 
to each other, the vaster the scale on which tides 
will be formed. If the bodies are liquid or gaseous, 
they will be distorted by the force of gravitation, and 
the figure of both bodies will become ellipsoidal ; and 
at last under greater stress, the restraining shell of 
both bodies will burst asunder on opposite sides in 
streams of matter from the interior. In this 
manner the arms of the spiral are formed. 

As Chamberlin puts it: "If, with these potent 
forces thus nearly balanced, the sun closely ap- 
proaches another sun, or body of like magnitude . . . 
the gravity which restrains this enormous elastic 
power will be reduced along the line of mutual at- 
traction. At the same time the pressure transverse 
to this line of relief will be increased. Such local- 
ized relief and intensified pressure must bring into 
action corresponding portions of the sun's elastic 
potency, resulting in protuberances of correspond- 
ing mass and high velocity." 

Only a fraction of one per cent of the sun's mass 
ejected in this fashion would be sufficient to gener- 
ate the entire planetary system. Nuclei or knots in 
the arms of the spiral gradually grew by accretion, 
the four interior knots forming Mercury, Venus, the 
Earth, and Mars. The earth knot was a double one, 
which developed into the earth-moon system. The 
absence of a dominating nucleus beyond Mars ac- 
counts for the zone of the asteroids remaining in 
some sense in the original planetesimal condition. 
The vaster nuclei beyond Mars gradually condensed 
into Jupiter, Saturn, Uranus, and Neptune; and 



374 ASTRONOMY TO-DAY 

lesser nuclei related to the larger ones form the 
systems of moons or satellites. 

The orbits of the planetesimals and the planetary 
and satellite nuclei would be very eccentric, forming 
a confusion of ellipses with frequently crossing 
paths. Collisions would occur, and the nuclei would 
inevitably grow by accretion. Each planet, then, 
would clear up the planetesimals of its zone; and 
Moulton shows that this process would give rise to 
axial revolution of the planet in the same direction 
as its orbital revolution. The eccentricities would 
finally disappear, and the entire mass would revolve 
in a nearly circular orbit. 

Rotation twists the streams into the spiral form, 
and the huge amounts of wreckage from the near- 
collision are thrown into eddies. The fragments or 
particles (planetesimals) which have given the 
name to the theory, begin their motion round 
their central sun in elliptical paths as required by 
gravitation. The form of the spiral is preserved by 
the orbital motion of its particles. There is a 
gradual gathering together of the planetesimals at 
points or nodes of intersection, and these become 
aggregations of matter, nuclei that will perhaps be- 
come planets, though more likely other stars. The 
appulse or near approach is but one of the methods 
by which the spiral nebulae may have come into ex- 
istence. The planetesimal hypothesis would seem 
to account for the formation of many of these ob- 
jects as we see them in the sky, though perhaps it is 
hardly competent to replace entirely the Laplacian 
hypothesis of the formation of the solar system, 
which would appear to be a special case by itself. 

It will be observed that while the Laplacian hy- 
pothesis is concerned in the main with the pro- 



COSMOGONY 875 

gressive development of the solar system, and 
systems of a like order surrounding other stellar 
centers, whose existence is highly probable, the 
origin and development of the stellar universe is a 
vaster problem which can only be undertaken and 
completed in its broadest bearings when the struc- 
ture of the stellar universe has been ascertained. 

Darwin's important investigations in 1877-1878 
on tidal friction may be here related. Before his day 
acceptance of the ring-theory of development of the 
moon from the earth had scarcely been questioned ; 
but his recondite mathematical researches on the 
tidal reaction between a central yielding mass and 
a body revolving round it brought to light the un- 
suspected effect of tides raised upon both bodies by 
their mutual attraction. The type of tides here 
meant is not the usual rise and fall of the waters of 
the ocean, but primeval tides in the plastic material 
of which the earth in its early history was composed. 
The Newtonian law of gravitation afforded a com- 
plete explanation of the rise and fall of the waters 
of the oceans, but as applied to the motions of 
planets and satellites by the Lagrangian formulse, it 
presupposed that all these bodies are rigid and un- 
yielding. However, mutual tides of phenomenal 
height in their early plastic substances must have 
been a necessary consequence of the action of the 
Newtonian law, and they gradually drew upon the 
earth's rotational moment of momentum. 

In its very early history, before there was any 
moon to produce tides, the earth rotated much more 
rapidly, that is, the day was very much shorter than 
now, probably about five or six hours long. And 
with the rapid whirling, it was not a Laplacian 
ring that was detached, but a huge globular mass 



376 ASTRONOMY TO-DAY 

was separated from the plastic earth's equator. 
Darwin shows that the ^ravitative interaction of the 
two bodies immediately began to raise tides of ex- 
traordinary height in both, therefore tending to 
slow down the rotational periods of both bodies. 
Action and reaction being equal, the reaction at once 
began driving the moon away from the earth and 
thereby lengthening its period of revolution. So 
small was the mass of the moon and so near was it 
to the earth, that its relative rotational energy was 
in time completely used up, and the moon has ever 
since turned her constant face toward us. Tides 
of sun and moon in the plastic earth, acting through 
the ages, slowed down the earth's rotation to its 
present period, or the length of the day. 

Moulton, however, has investigated the tidal the- 
ory of the origin of the moon in the light of the 
planetesimal hypothesis, concluding that the moon 
never was part of the earth and separated there- 
from by too rapid rotation of the earth, but that 
the distance of the two bodies has always been the 
same as now. The more massive earth has in its 
development throughout time robbed the less mas- 
sive moon in the gradual process of accretion. So 
the moon has never acquired either an ocean or at- 
mosphere, and this view is acceptable to geologists 
who have studied the sheer lunar surface, Shaler of 
Harvard among the first, and laid the foundations 
for a separate science of selenology. 

Tidal friction has also been operant in producing 
sun-raised tides upon the early plastic substances 
which composed the planets : more powerfully in the 
case of planets nearer the sun; less rapidly if the 
planet's mass is large; also less completely if the 
planet has solidified earlier on account of its small 



COSMOGONY 377 

dimensions. So Darwin would account for the 
present rotation periods of all the planets: both 
Mercury and Venus powerfully acted on by the sun 
on account of their nearness to him, and their rota- 
tional energy completely exhausted, so that they now 
and for all time turn a constant face toward him, 
as the moon does to the earth; earth and possibly 
Mars even yet undergoing a very slight lengthening 
of their day; Jupiter and Saturn, also Uranus and 
probably Neptune, still exhibiting relatively swift 
axial rotation, because of their great mass and 
great original moment of momentum, and also by 
reason of their vast distances from the central tide- 
raising body, the sun. 

By applying to stellar systems the principles de- 
veloped by Darwin, See accounted for the fact, to 
which he was the first to direct attention, that the 
great eccentricity of the binary orbits is a necessary 
result of the secular action of tidal friction. The 
double stars, then, were double nebulae, originally 
single, but separated by a process allied to that 
known as "fission" in protozoans. Indeed, Poin- 
care proved mathematically that a swiftly revolving 
nebula, in consequence of contraction, first under- 
goes distortion into a pear-shaped or hour-glass 
figure, the two masses ultimately separating en- 
tirely; and the observations of the Herschels, Lord 
Rosse and others, with the recent photographic 
plates at the Lick and Mount Wilson observatories, 
afford immediate confirmation in a multitude of 
double nebulae, widely scattered throughout the 
nebular regions of the heavens. 

Jeans of Cambridge, England, among the most 
recent of mathematical investigators of the cosmog- 
ony, balances the advantages and disadvantages of 



378 ASTRONOMY TO-DAY 

the differing cosmogonic systems as follows, in his 
"Problems of Cosmogony and Stellar Dynamics": 
"Some hundreds of millions of years ago all the 
stars within our Galactic universe formed a single 
mass of excessively tenuous gas in slow rotation. 
As imagined by Laplace, this mass contracted owing 
to loss of energy by radiation, and so increased its 
angular velocity until it assumed a lenticular shape 
...... After this, further contraction was a sheer 

mathematical impossibility and the system had to 
expand. The mechanism of expansion was pro- 
vided by matter being thrown off from the sharp 
edge of the lenticular figure, the lenticular center 
now forming the nucleus, and the thrown-off matter 
forming the arms, of a spiral nebula of the normal 
type. The long filaments of matter which consti- 
tuted the arms, being gravitationally unstable, first 
formed into chains of condensation about nuclei, 
and ultimately formed detached masses of gas. 
With continued shrinkage, the temperature of these 
masses increased until they attained to incandes- 
cence, and shone as luminous stars. At the same 
time their velocity of rotation increased until a 
large proportion of them broke up by fission into 
binary systems. The majority of the stars broke 
away from their neighbors and so formed a cluster 
of irregularly moving stars — our present Galactic 
universe, in which the flattened shape of the original 
nebula may still be traced in the concentration 
about the Galactic plane, while the original motion 
along the nebular arms still persists in the form of 
'star-streaming.* In some cases a pair or small 
group of stars failed to get clear of one another's 
gravitational attractions and remain describing or- 
bits about one another as wide binaries or multiple 



COSMOGONY 379 

stars. The stars which were formed last, the pre- 
sent B-type stars, have been unusually immune from 
disturbance by their neighbors, partly because they 
were born when adjacent stars had almost ceased 
to interfere with one another, partly because their 
exceptionally large mass minimized the effect of 
such interference as may have occurred; conse- 
quently they remain moving in the plane in which 
they were formed, many of them still constituting 
closely associated groups of stars — the moving star 
clusters. 

"At intervals it must have happened that two 
stars passed relatively near to one another in their 
motion through the universe. We conjecture that 
something hke 800 million years ago our sun ex- 
perienced an encounter of this kind, a large star 
passing within a distance of about the sun's diame- 
ter from its surface. The effect of this, as we have 
seen, would be the ejection of a stream of gas to- 
ward the passing star. At this epoch the sun is 
supposed to have been dark and cold, its density 
being so low that its radius was perhaps compar- 
able with the present radius of Neptune's orbit. The 
ejected stream of matter, becoming still colder by 
radiation, may have condensed into liquid near its 
ends and perhaps partially also near its middle. 
Such a jet of matter would be longitudinally un- 
stable and would condense into detached nuclei 
which would ultimately form planets." 



CHAPTER LXI 

COSMOGONY IN TRANSITION 

WE have seen how Wright in 1750 initiated a 
theory of evolution, not only of the solar 
system, but of all the stars and nebulae as well ; how 
Kant in 1752 by elaborating this theory sought to 
develop the details of evolution of the solar system 
on the basis of the Newtonian law, though weak- 
ened, as we know, by serious errors in applying 
physical laws ; how Laplace in 1796 put forward his 
nebular hypothesis of origin and development of 
the solar system, by contraction from an original 
gaseous nebula in accord with the Newtonian law; 
how Sir William Herschel in 1810 saw in all nebulae 
merely the stuff that stars are made of; how Lord 
Rosse in 1845 discovered spiral nebulae ; how Helm- 
holtz in 1854 put forward his contraction theory of 
maintenance' of the solar heat, seemingly reinforc- 
ing the Laplacian theory ; how Lane in 1870 proved 
that a contracting gaseous star might rise in tem- 
perature; how Roche in 1873 in attempting to 
modify the Laplacian hypothesis, pointed out the 
conditions under which a satellite would be broken 
up by tidal strains; how Darwin in 1879 showed 
that the theory of tidal evolution of non-rigid bodies 
might account for the formation of the moon, and 
binary stars might originate by fission ; how Keeler 
in 1900 discovered the vast numbers of spiral ne- 
bulae; how Chamberlin and Moulton in 1903 put f or- 

880 



COSMOGONY IN TRANSITION 881 

ward the planetesimal hypothesis of formation of 
the spiral nebulse, showing also how that hypothesis 
might account for the evolution of the solar system ; 
and how Jeans in 1916 advocated the median ground 
in evolution of the arms of the spiral nebulse, show- 
ing that they will break up into nuclei, if 
sufficiently massive. 

In all these theories, truth and error, or lack 
of complete knowledge, appear to be intermingled 
in varying proportions. Is it not early yet to say, 
either that any one of them must be abandoned as 
totally wrong, or on the other hand that any one of 
them, or indeed any single hypothesis, can explain 
all the evolutionary processes of the universe? 

Clearly the great problems cannot all be solved by 
the kinetic theory of gases and the law of gravita- 
tion alone. Recent physical researches into sub- 
atomic energy and the structure and properties of 
matter, appear to point in the direction where we 
must next look for more light on such questions as 
the origin and maintenance of the sun's heat, the 
complex phenomena of variable stars and the pro- 
gressive evolution of the myriad bodies of the stellar 
universe. Because we have actually seen one star 
turn into a nebula we should not jump to the con- 
clusion that all nebulse are formed from stars, even 
if this might seem a direct inference from the high 
radial velocities of planetary nebulse. 

Quite as obviously many of the spiral nebulse are 
in a stage of transition into local universes of stars 
— even more obvious from the marvelous photo- 
graphs in our day than the evolution of stars from 
nebulse of all types was to Herschel in his day. 

The physicist must further investigate such ques- 
tions as the building up of heavy atomic elements 



382 ASTRONOMY TO-DAY 

by gravitative condensation of such lighter ones as 
compose the nebulae; and laboratory investigation 
must elucidate further the process of development of 
energy from atomic disintegration under very high 
pressures. This leads to a reclassification of the 
stars on a temperature basis. 

Equally important is the inquiry into the me- 
chanism of radiative equilibrium in sun and stars. 
Not impossibly the process of the earth's upper at- 
mosphere in maintaining a terrestrial equilibrium 
may afford some clue. What this physical mech- 
anism may be is very incompletely known, but it 
is now open to further research through recent prog- 
ress of aeronautics, which will afford the investi- 
gator a "ceiling" of 50,000 feet and probably more. 
Beneath this level, perhaps even below 40,000 feet, 
lie all the strata, including the inversion layer, 
where the sun's heat is conserved and an equilib- 
rium maintained. 

Even ten years ago, had an astronomer been 
asked about the physical condition of the interior of 
the stars, he would have replied that information of 
this character could only be had on visiting the 
stars themselves — and perhaps not even then. But 
at the Cardiff meeting of the British Association in 
1920, Eddington, the president, of Section A, de- 
livered an address on the internal constitution of the 
stars. He cites the recent investigations of Russell 
and others on truly gaseous stars, like Aldebaran, 
Arcturus, Antares and Canopus, which are in a 
diffuse state and are the most powerful light-givers, 
and thus are to be distinguished from the denser 
stars like our Sun. The term giants is applied to 
the former, and dwarfs to the latter, in accord with 
Russell's theory. 



COSMOGONY IN TRANSITION 383 

As density increases through contraction, these 
terms represent the progressive stages, from earlier 
to later, in a star's history. A red or M-type star 
begins its history as a giant of comparatively low 
temperature. Contracting, according to Lane's law, 
its temperature must rise until its density becomes 
such that it no longer behaves as a perfect gas. 
Much depends on the star's mass; but after its 
maximum temperature is attained, the star, which 
has shrunk to the proportions of a dwarf, goes on 
cooling and contracts still further. 

Each temperature-level is reached and passed 
twice, once during the ascending stage and once 
again in descending — once as a giant, and once as a 
dwarf. Thus there are vast differences in lumin- 
osity: the huge giant, having a far larger surface 
than the shrunken dwarf, radiates an amount of 
light correspondingly greater. 

The physicist recognizes heat in two forms — ^the 
energy of motion of material atoms, and the energy 
of ether waves. In hot bodies with which we are 
familiar, the second form is quite insignificant ; but 
in the giant stars, the two forms are present in about 
equal proportions. The super-heated conditions of 
the interior of the stars can only be estimated in 
millions of degrees; and the problem is not one of 
convection currents, as formerly thought, bringing 
hot masses to the surface from the highly heated 
interior, but how can the heat of the interior be 
barred against leakage and reduced to the rela- 
tively small radiation emitted by the stars. 
"Smaller stars have to manufacture the radiant 
heat which they emit, living from hand to mouth; 
the giant stars merely leak radiant heat from their 
store." 



384 ASTRONOMY TO-DAY 

So a radioactive type of equilibrium must be estab- 
lished, rather than a eonvective one. Laboratory- 
investigations of the very short waves ar6 now in 
progress, bearing on the transparency of stellar ma- 
terial to the radiation traversing it; and the pene- 
trating power of the star's radiation is much like 
that of X-rays. The opacity is remarkably high, 
explaining why the star is so nearly "heat-tight." 

Opacity being constant, the total radiation of a 
giant star depends on its mass only, and is quite 
independent of its temperature or state of diffuse- 
ness. So that the total radiation of a star which is 
measured roughly by its luminosity, may readily re- 
main constant during the entire *giant' stage of its 
history. As Russell originally pointed out, giant 
stars of every spectral type have nearly the same 
luminosity. From the range of luminosity of the 
giant stars, then, we may infer their range of 
masses: they come out much alike, agreeing well 
with results obtained by double-star investigation. 

These studies of radiation and internal condition 
of the stars again bring up the question of the origi- 
nal source of that supply of radiant energy con- 
tinually squandered by ail self-luminous bodies. 
The giant stars are especially prodigal, and radiate 
at least a hundredfold faster than the sun. 

"A star is drawing on some vast reservoir of 
energy," says Eddington, "by means unknown to 
us. This reservoir can scarcely be other than the 
sub-atomic energy which, it is known, exists abun- 
dantly in all matter ; we sometimes dream that man 
will one day learn how to release it and use it for his 
service. The store is well-nigh inexhaustible, if only 
it could be tapped. There is sufficient in the sun to 
maii\tain its output of heat for fifteen billion years." 



r-L. 



m:: 



i\* 



Mil! 



^mm^ 



a 



I -^ 



M 



UBRARVOFO 



.ONGRE^,, 




003 642230 



LIBRARY OF CONGRESS 



028 163 129 9 



